The create-cohort command creates a set of cohort keys for a given set of snaps.
A cohort is a view or snapshot of a snap's "channel map" at a given point in
time that fixes the set of revisions for the snap given other constraints
(e.g. channel or architecture). The cohort is then identified by an opaque
per-snap key that works across systems. Installations or refreshes of the snap
using a given cohort key would use a fixed revision for up to 90 days, after
which a new set of revisions would be fixed under that same cohort key and a
new 90 days window started.
