What determines the numbering sequence of Mail Data folders in ~/Library?

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What determines the numbering sequence of Mail Data folders in ~/Library

Can anyone explain the process behind the numbering sequence of the folders holding emlx files in ~/Library/Mail/V2/Mail Account name/mbox/Folder with alphanumeric name/Data/0 (where 0 is the first in a sequence of numbered folders all on the same directory)?
Also, within each numbered folder there are further nested folders also with numbered names. i.e. 0,1,2 etc. At the bottom of the directory resides a folder Messages containing the emlx files.