LDInspector

[N. W. Perry]

For the subfiles I understand the spec (here) 

Code:

<Optional Qualifier> is a sequential number, starting with 1, added if there is more than one set that could be assigned <Set Number>.

that the "-1" would be required if there is another set with that number, but not added if there is no other set with that number. So at the time of modelling the first set, it seems to be kind of "not forbidden" and "not required" (how could I know if there would be another set with that number in the future?). It well may be another misunderstanding of mine.  I will happily adopt the check as soon as I have a new understanding.  

That's right, but for the base filename (the MPD as a whole), the spec is different:

Each MPD for the set will be named in the following manner:
<Set Number>[-<Optional Qualifier>] - <Set Name>[ - <Sub Model Name>]

Where:
<Optional Qualifier>: Is a sequential number, starting with 2.

The <Optional Qualifier> is not mandatory and gets added only if there is more than one set that could be assigned the same <Set Number>. The first set using a given number would be understood to never contain the qualifier however numbering should start with the oldest set and some investigation should be done in existing set databases.

So it's pretty clear that the intent is not to use 1 at all in the base filename, even when the set is the first of several with the same number. But, as you've quoted, the qualifier in subfile names does start at 1, not 2, and that it's "added if there is more than one set that could be assigned <Set Number>."

But as you also say, it seems reasonable that the start of every subfile name should be identical to that of the main file, and I have a feeling this is the intent of the specification. So it might be worth clarifying with the Standards Board (which I've now done).