Bottom bracket length query

[Once A Hero]

Looking at using an M900 XTR bottom bracket paired with M900 XTR cranks on a build, but wondered how you determine which BB length is the best to use of the 2 offered; 107mm or 113mm?

What drives the need or not for the 6mm difference?

And is the difference all on one side of the BB or shared between each side?

 

[bikeworkshop]

Chainstay design can require a longer bb - think aluminium frame with wider tyre clearance vs steel

Fat seattubes can also mean a longer bb is required for the front derailleur to drop the chain onto the small ring

Pedaling is best narrower.

 

[Once A Hero]

Chainstay design can require a longer bb - think aluminium frame with wider tyre clearance vs steel

Fat seattubes can also mean a longer bb is required for the front derailleur to drop the chain onto the small ring

Is there a table or directive of some sort that says "if seat tube = X diameter then use xmm bottom bracket"?

Can't find anything clear from Shimano as to why they offered two lengths and when they should be applied.

I had an old Klein Rascal and have a similar vintage Marin both with fat seat tubes taking a 34.9 dia front mech, both running XTR gear, and the Klein at least had a 107mm bb but feels like it would have been an ideal candidate for a 113mm.