For instance there are 3 cylinders SKD1. The will have a unique bitting.
Then there are 7 cylinders SKD2. The will have a unique bitting.
SKD1 and SKD2 are groups keyed differently, are a Level 1 system (only one change key operates them).
Regarding of how many cylinders are in an SKD_ group those cylinders are always keyed alike and never operate or are operated by a key from a different SKD group.
SKD is defined in several places such as Key Systems and Nomenclature from DHI:
Download File
and UL437 as well
All SKD1s are keyed alike
All SKD2s are keyed alike
and so on...