I think the choreo sequence, at least in singles, is a substitute for the old 6.0 long program requirement for "moves in the field."
I think, and a historian will correct me if I'm wrong, is that Moves in the Field was the one and only requirement in the 6.0 Free Program. Otherwise, skaters could do whatever elements they wanted... in theory.
As I recall (i.e., I may be mistaken on details)...
Originally the free program was completely free/had no requirements, although there were expectations that it would include a variety of jumps, spins, steps, and other skating movements.
First came the Zayak rule, which limited the number of times one could repeat the same jump and how many could be repeated at all, originally just applied to triple jumps, later applied to quads as well, and later still related but less strict limitations on repeating double axels and then all doubles.
Aside from the Zayak requirements of repeated jumps being done in combination, by the early 1990s there was a requirement of at least one jump combination or sequence, later with a limit of maximum three combos/sequences.
By the late 1990s there were "well-balanced program" expectations -- it was explicitly recommended that free programs include a minimum number of spins, a full step sequence, and either a second step sequence (men) or spiral sequence (women and pairs)
Then around the turn of the century these expectations were codified into well-balanced program
requirements, and it was also at/around this time that the second sequence was described as "moves in the field" or "field moves," to include spread eagles, Ina Bauers, etc., in addition to spirals (since spirals were required for women, the idea seems to have been to give different options to men to maintain a gender difference)
As of the end of the 6.0 era, these requirements were
minimums, such that a program that was missing a fourth spin or any step sequence or spiral/field moves sequence could be penalized as lacking. Not that it was clear whether judges were penalizing, since there were only two numbers from each judge for the whole program that didn't break anything down.
There weren't any maximums for these elements, or for jump passes.
Then came IJS. To prevent skaters from building up points just based on quantity of elements (as opposed to difficulty or quality) and also to prevent do-overs if a skater missed a jump or other element and wanted to try again for full credit, IJS redefined the well-balanced program requirements in terms of
maximum number of each type of element one could get credit for. There were maximum numbers of slots for jumps, spins, step sequence (two of different shapes for men), and spiral sequence for women (and various pair and dance elements in those disciplines), and requirements for certain kinds of elements within the broader category (at least one/no more than two jump combos/sequences at first but soon a third was allowed again; only two jumps per combo/sequence until one three-jump combo/sequence was soon allowed; at least one axel takeoff; a combination spin, a flying spin, a spin in one position). Of course there was incentive to fill all the slots because leaving out an element/leaving a slot empty meant a lost opportunity to gain points, but there was no other penalty for doing fewer jumps or spins or not doing a step sequence.
But it was often not worth trying to make up for a missed jump or missed spin because of the limited number of slots and requirements for certain kinds of jump and spin elements to fill them. Trying to do a high-value element in the next slot might end up not being valid, and worse yet it could also invalidate another later element (e.g., saving the only axel for the last jump element and then because of adding a makeup doing a non-axel jump as the seventh jump pass when only seven are allowed so that one doesn't count because it needed to be an axel, and the eighth jump pass won't count either with only seven jump slots available).
There were originally a total of 14 elements for men (8 jump passes, 4 spins, 2 step sequences of different shapes) and 13 for women (7 jump passes, 4 spins, 1 step sequence, 1 spiral sequence).
After a couple of years, the ISU decided that the programs were too full of elements and decided to eliminate the fourth spin originally allowed in senior free skates, and to eliminate the spiral/second step sequence in junior free skates which were 30 seconds shorter and already only had 3 spins. So now senior men's free skates had 13 elements and women's had 12 -- 12 and 11 respectively for juniors.
A few years after that, they removed the women's spiral sequence and men's second step sequence from short programs and replaced both in the free skate with a choreographic sequence (which originally was required to include at least one brief spiral position for women). Also, step sequences no longer had specified shapes. This didn't change the total number of free skate elements but it did replace one leveled element with a choreo element.
Then the men's free skate was reduced by 30 seconds to match the women's free skate length (4:00 for seniors, 3:30 for juniors), and one of the jump elements was deleted so now men also could do only 7 jump passes.
And around that time, maybe a year or two earlier or later, the junior freeskate step sequence was changed to a choreo sequence -- juniors still do leveled steps in short programs, while senior free skates still include both leveled steps and choreo sequences.
Starting next year, both men and women will lose another jump pass so there will be a maximum of 6 jump slots and a total of 11 elements in seniors, 10 for juniors (since juniors only have one sequence).
The idea, as I understand it, is to give more time for choreography to be evaluated in the program components instead of just as scored elements.
We'll have to wait to see if there will be other changes to the free skate/well-balanced program rules for 2026-27 and beyond.