The Active set is the subset of the Full set of dancers who are actually performing in the current Repeat of the dance. It is not in common use but is used in this Dictionary of Scottish Country Dancing wherever the Beginner might be confused by the expression, The set, which is loosely used to mean either the Full set or the Active set.
This differentiation is particularly necessary when dealing with Longwise, Full sets of 4 Couples in which either 2 or 3 Couples are actually performing (as the Active set(s)) while others are Standing, awaiting the next Repeat; it is also relevant for Round the room sets for which there is an odd Partner group of Couples or threesomes.
Whenever any Partner group is identified by Place number as, for example, 2nd couple of the Active set, that means the Place in the Active set occupied at the beginning of the current Repeat.
For more details, see 3 Couple repeat in 4 couple set, 2 Couple repeat in 4 couple set and Round the room sets in Types of sets.