Decision reason and source

An Assignment carries two enums. They answer different questions and vary independently, so neither can be derived from the other.

Question	Field	Values
Why this variant rather than the hash bucket?	reason (DecisionReason)	assigned, forced, fallback_disabled, fallback_targeting_mismatch
Where did the value come from?	source (AssignmentSource)	computed, store

use Rasuvaeff\Yii3AbTesting\AssignmentSource;
use Rasuvaeff\Yii3AbTesting\DecisionReason;

$assignment->reason === DecisionReason::FallbackTargetingMismatch;
$assignment->source === AssignmentSource::Store;

$assignment->isForced();
$assignment->isFallback();           // either fallback reason
$assignment->isTargetingMismatch();
$assignment->isSticky();             // source === store

Why two axes

With a single "fallback" flag, two very different situations look the same: an experiment someone switched off, and a subject outside the target segment. The first is an operational event; the second is normal traffic that was never eligible. Reports and alerts need to tell them apart.

The source axis is separate because a stored value can carry any reason. A forced variant served back from a sticky cookie is forced + store — a legitimate combination that one combined field could not express.

Only assigned is participation

Reason	Counts in analysis	Typical cause
assigned	yes	The subject was bucketed by the hash
forced	no	QA or a support override
fallback_disabled	no	The kill switch is off
fallback_targeting_mismatch	no	The subject did not match the targeting rules

DecisionReason::isAnalyzable() states the rule in code, and both ExposureEvent and ConversionEvent expose it as isAnalyzable(). Every query in Reading results filters on decision_reason = 'assigned'.

Events are still recorded for the other reasons. Excluding them at write time would make an accidental kill switch or a broken targeting rule invisible in the very data you would use to notice it.

On the wire

Both values travel in every event row as decision_reason and assignment_source — see Events v2.