2.1.1731 Part 4 Section 6.2.2.13, idmap (Shape ID Map)

a.   The standard allows for application-dependent behavior.

Office ignores this value on file read. The value indicates ID blocks of 1024 values, where a value of 0 means IDs 0 to 1024, a value of 1 means IDs 1025 to 2048, and so on. If Office has at least one shape which has an ID contained in a specific block, it writes out the index for that block.