Subset Settings Dialog Box

Click the Settings button at the bottom of the Navigator palette.
Choose the Subset Settings command in the context menu opened by right-clicking (or Ctrl-clicking) the name of the Subset.
Do not include this Subset in ID sequence: Check this box to exclude the Subset from automatic ID assignment. (If you check this box, the following option for automatic ID assignment is grayed.)
Assign ID Automatically: Choose this option to assign an ID automatically, based on the Prefix and Style defined in Book Settings.
Custom ID: Choose this option to assign a Subset ID of your choice.
Continue Using ID Assignment of Upper Levels: Check this option if you want Layouts within this Subset to be assigned IDs as if they were not within this Subset, but rather part of the level above. In this case you are only using the Subset as a logical grouping which has no effect on IDs.
Customize ID assignment: Check this option if you want the Layouts inside the Subset to be assigned custom IDs:
Note: When IDs by letters exhausts the alphabet, the numbering continues as follows: y, z, aa, ab, ac. (The initial letter in double-letter numbering always consists of a single character regardless of the language being used.)