In the Tooling Dept of a large aerospace company I used to work for, they used the following (Prior to V5):
-703 Assy consists of detail parts -704, -705 & -706
-707 Assy consists of detail parts -708, -709, -710 & -711
Though this kind of makes sense, it could get confusing sometimes, and, doesn't lend itself to revisions very well.
If the -703 Assy was revised, you had to come up with a new number that is out of sequence.
Then we got V5 and tried to come up with new numbering.
They kind of settled on the following:
-1 Assy consists of detail parts -100 and on, up to 199
-2 Assy consists of detial parts -200 and on, up to 299
A little better, but still has some problems.
Some of us proposed the following:
-100 Assy consists of detail parts -101 and on, up to -199
-200 Assy consists of detail parts -201 and on, up to -299
If assy needed more than 198 unique detail parts:
-1000 Assy consists of detail Parts -1001 and on, up to 1999