If your line has unique numbers for structures, I believe you put your number in the Comment #1 field and tell PLS-CADD to use that for the structure number otherwise you get a sequential number starting with 1. If you need more help, and are a dues paying customer, call their Tech Support. Most of the lines I look at already have the structure number set in Comment #1 field. I don't remember right now where the setting is but the manual may help.
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I have been called "A storehouse of worthless information" many times.