(colloquial) Used when a speaker attempts to categorize an observable fact or event, although the category is subjective or lacks clearly defined parameters.

(informal) Used as a farewell, when the next time the speaker and interlocutor will meet is not known.

(strictly) When in a foreign place, suit behavior or appearance to the local culture. / (loosely, idiomatic) Adapt to the circumstances; follow common custom.

(idiomatic, by ellipsis) Adjust to local customs.

(uncommon) Alternative form of when in Rome, do as the Romans do

(uncommon) Alternative form of when in Rome, do as the Romans do

When faced with a difficult decision, it is better not to take unnecessary risks. Also when in doubt, abstain and when in doubt, leave it out