Underscore file ids?

[oconnor0]

In https://github.com/seaweedfs/seaweedfs/wiki/Master-Server-API#assign-a-file-key, when describing passing a count in to assign multiple file ids, there is a description of "underscore addressing":

how many file ids to assign. Use <fid>_1, <fid>_2 for the assigned additional file ids. e.g. 3,01637037d6_1, 3,01637037d6_2

I don't seem to be able to find documentation on this. As near as I can tell, this effectively extends the size of a file id from 16-bytes to 24-bytes as it appears to be parsed as an 64-bit unsigned integer.

Is this correct?

Also, what are the consequence of these kind of ids? If I write <fid0> and then after many other unrelated writes to the same volume write <fid0>_1, are there segmentation/fragmentation/performance issues?