Yii version numbering
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A.B.C

A = For Yii2 it's always 2.
B = Major version. Non-BC changes with upgrade instructions.
C = BC changes and additions.

A.B.CrcD

This is when we want to release release candidate. D is the RC number. Starts with 1 and increments till we're getting a stable release with no critical bugs and BC incompatibility reports.