The explanation is in SRCREV_FORMAT

As described here,

SRCREV_FORMAT helps construct valid SRCREV values when multiple source controlled URLs are used in SRC_URI.

The system needs help constructing these values under these circumstances. Each component in the SRC_URI is assigned a name and these are referenced in the SRCREV_FORMAT variable. Consider an example with URLs named “machine” and “meta”. In this case, SRCREV_FORMAT could look like “machine_meta” and those names would have the SCM versions substituted into each position. Only one AUTOINC placeholder is added and if needed. And, this placeholder is placed at the start of the returned string.