Nothing, really.
But the ISU wants to have some standardized steps that everyone has to do, so the can compare apples to apples for a part of the dance, the last remainder of the separate compulsory pattern dances.
I.e., the juniors have to do
these exact steps
twice through (not necessarily consecutively), switching roles so that the man skates the woman's steps and vice versa one of those times.
The seniors just need to do a subset of the specified edges in that pattern, and in between they're supposed to make up their own steps to connect steps 16 and 26 of the set pattern.
The point is that everyone has to do that exact sequence of edges in that pattern, with each edge held for the specified number of counts.
That pattern was choreographed in the 1930s to be skated to Paso Doble music and has been used as a set pattern/compulsory dance to paso doble music ever since. The name of the pattern is Paso Doble.
However, for this Rhythm Dance, the skaters are supposed to use those steps but they're supposed to skate it to "any dance style with the range of tempo: 56 measures of 2 beats per minute" (juniors) or "any dance style – min 110 beats per min, in 2/2, 2/4 or 4/4 time" (seniors). The dance style chosen needs to fit into the “Social Dances and Styles of the 1950s, 1960s and 1970s” theme. Whatever music that the skaters choose needs to fit that timing and fit the Rhythm Dance theme. It should NOT be a Paso Doble music. The skaters should interpret the music that they choose according to appropriate interpretation of those 1950s, 60s, 70s dance styles. They just need to use the specified steps that are also used to paso doble music in a different context.
They'll have to find ways to do those moves in holds and/or to include them in the portions of the program where they are allowed/required not to be touching, e.g., the Step Sequence Not Touching.
For that reason, it would be unwise for teams to choose a Twist theme for the entire dance. We'll get medleys.
They want social dance of the 1950s-60s era, with rhythms that were characteristic of those eras and aren't part of the standard "ballroom dance" syllabus. They don't want waltzes and foxtrots. They do want social dance, but the kinds of dances that were popular during the specified eras.
As mentioned above, it's so that everyone will be doing the exact same steps for part of the dance, as a point of direct comparison.
Rather than making up a new sequence of steps that everyone has to do, they're choosing a sequence that all the ice dancers already know and telling them to repurpose these steps to a different kind of music.
Same as the seniors used steps from the Silver Samba last year when skating to 1980s music. They weren't supposed to be dancing a samba, they were supposed to be expressing whichever piece of 80s popular music that they chose. But the specific required steps were taken from a dance that was originally choreographed as a samba.
That's a little tricky.
In the specified eras, it became common for popular dancing to be done as individuals dancing solo or in groups rather than in dance holds as couples. So any kinds of dances that were popular at dances, parties, dance clubs/discos, etc., of that era are what is wanted.
However, ice dance requires couples skating in connection with each other for most of the program.
So the teams will need to find ways to interpret in dance holds dance styles that might have been more often danced solo or in larger groups at the times they were popular.
The ISU cares when it comes to the pattern dance portion of the RD because the requirements for the set pattern steps are that the edges are each held for a set amount of time. If the tempo were significantly slowed down (or sped up), the teams would not be demonstrating the same mastery of those steps and their respective timing.
For the rest of the dance, the non-pattern-dance elements and the in-between skating, the beats per minute are not specified.
The theme of this year's RD is social dances of the 1950s, 60s, and 70s.
That's less specific than, e.g., "Latin dance" or "Memories of a Grand Ball."
As you point out, latin dances like mambo were popular during that era, so I won't be surprised if we see some.
The competitive teams and the younger coaches may need to consult movies for examples of how people were dancing at parties and clubs in those eras. Older coaches may remember their own social dance experience from their youth.