Just as there are "levels" in square dancing so there are "phases" (expressed in roman numbers I to VI) in round dancing. Again as in square dancing, the PHASES are simply lists of figures. Here is an example:
TWO-STEP - PHASE III
Rating a particular dance is quite easy: if the dance consists of figures all belonging to Phase II, for instance, it is a Phase II dance. If all the figures belong to Phase Il except one which is in Phase III, the dance is rated II+1. If there are two Phase III figures, the dance will be rated II+2. BUT ! if there are at least 3 Phase III figures in the dance . . it is a Phase III dance. Let's go one step further in complexity: suppose a dance including mostly Phase III waltz figures, one Phase IV figure and one Phase VI figure (an extreme case just for the sake of argument). Well, the dance would be rated Phase V+1! That's the way it goes: one single phase VI figure causes the whole dance to be rated at the Phase immediately under that figure. The fact that there is not a single Phase V figure in the dance is "starling snot" as they say in this country -- meaning insignificant.
All the rhythms used in round dancing contain figures belonging to several phases, BUT ! all the phases are not represented in all the rhythms. For instance: there is no such thing as a Phase IV Two-step. All Two-step figures are contained in phases I to III. Similarly, there is no such thing as a Phase II cha cha. Cha cha figures start at Phase III, as well as Rumba, Mambo, Tango, Jive, Quickstep, Foxtrot and Slow Two-step figures. There are even two rhythms, Samba and Paso doble, that start at Phase IV. The only rhythm in which all phases are represented is the Waltz. To be complete in this overview, I should also mention that there are not only figures rated in the Roundalab rating system but also "actions" and "movements". An "action" is a motion without weight change, for instance "point". A "movement" is a motion requiring a change of weight, for instance "change point".
Again as in square dancing, the dance rating does not necessarily reflect the degree of difficulty of that dance. Just as there is easy Plus and tough Mainstream, there are easy Phase IV rounds and pretty tough Phase II dances (the waltz "Feelin" is a Phase II dance; "Old Vienna" and "Kontiki" are only rated II+1!).
The Roundalab rating system is now used everywhere with very few exceptions: the old terms "easy", "intermediate" and "advanced" have disappeared, which is why it is important that all round dancers understand the Phase system: know your "fluency phase" and avoid the disappointment of having to leave the floor because the dance is too hard.
The parallel with square dancing has often been drawn to facilitate this understanding, but keep in mind that this is only a very rough comparison. Phase II could be said to correspond, more or less, to Mainstream, Phase III to Plus, Phase IV to Advanced, Phase V to C1/C2 and Phase VI to C3/C4.
Understanding the Phase system is a good thing, becoming a "phase snob" is NOT. Remember that you will enjoy round dancing more, and you will look better on the floor, if you stay within your "comfort phase". In other words, a good Phase II dancer has more fun and looks better than a poor Phase IV performer.