Functional fallback
For example, a system can fall back from semantic to more exact matching when that yields a usable route.
Glossary
Fallback Search
Fallback Search is an alternative search route used when the primary search method does not provide a usable result or temporary no response.
Technical fallback
In the event of temporary failure, a controlled alternative can prevent the entire search experience from blocking.
Predictable behaviour
A fallback must be clearly limited so that visitors do not receive unexpected or misleading result sets.
Measurable
Register when fallback is used so that underlying issues do not remain invisible.