Auxiliary route
An auxiliary route is something users commonly do as part of a larger goal that isn't actually required for finishing it. Reading reviews while buying a car, using a meat thermometer while grilling, recognizing an account by photo before clicking on it: common enough to design around, but the larger goal would still count as done without them.
Definition
An auxiliary route is a route in-scope for a parent transition but not on its critical route — commonly present in actual instances, not causally required for parent-scale claim ratification.
Position relative to the critical route
The critical route is the causal-dependency map between committed starting conditions and committed completion criteria — what any route bridging those endpoints must satisfy. It's causally minimal given the endpoints, not descriptively complete. Auxiliary routes are the rest of what's in scope for the parent: user activity commonly observed in actual instances of the transition. They sit around the critical route as design considerations, not within its specification.
Auxiliary routes correspond to the Expected and Related tiers in condition tiering. The Essential tier operationalizes as completion criteria and produces the critical route; Expected and Related characteristics are structurally in scope for the parent but aren't definitional for the parent-scale claim. Frequency is the only distinction between the two tiers.
Described by every route primitive
Every route primitive applies to auxiliary routes — completion criteria, reachability, phenomenology. They differ from critical-route sub-transitions only in their structural role relative to the parent's scope-translation: the critical route's sub-transitions are required for parent-scale claim ratification; auxiliary routes aren't.
Looser ordering constraints
Because they aren't on the critical route's dependency chain, auxiliary routes typically have fewer ordering constraints. They can occur at various positions in the parent transition, can be skipped entirely, or can run in parallel with critical-route steps. Any dependencies they do have still govern — the critical route doesn't impose its own dependencies on them.
Example
A new user creating their first dashboard in a SaaS analytics tool. The critical route includes the sub-transitions causally required to produce "first dashboard created" from the committed starting conditions — selecting a data source, picking a chart type, naming the dashboard, saving. Watching the in-product onboarding video is in-scope for "first dashboard created" and common in actual instances, but isn't causally required to produce it. The user can create the dashboard without watching the video. Watching the video is an auxiliary route: a frequent part of how users traverse the parent transition, in scope for it but absent from the critical route.