A control set is the grouping of control signals (set/reset, clock enable and clock) that drives any given SRL, LUTRAM, or register. For any unique combination of control signals, a unique control set is formed. This is important, because registers within a 7 series slice all share common control signals, and thus, only registers with a common control set can be packed into the same slice. For example, if a register with a given control set has just one register as a load, the other seven registers in the slice it occupies will be unusable.
Designs with too many unique control sets might have many wasted resources as well as fewer options for placement, resulting in higher power and lower performance. Designs with fewer control sets have more options and flexibility in terms of placement, generally resulting in improved results.
In UltraScale™ devices, there is more flexibility in control set mapping within a CLB. Resets that are undriven do not form part of the control set, because the tie off is generated locally within the slice. However, it is good practice to limit unique control sets to give maximum flexibility in placement of a group of logic.