What is a UUID? Why is v4 the default? (with a look at v7)
Primary keys, request IDs, de-duplicated filenames — for most cases a plain UUID v4 is enough.
The scenario
A distributed system does not want auto-increment IDs and needs a globally unique identifier.
A quick look at the versions
v1 can leak node information; v4 is purely random and native to the browser; v7 is time-ordered but demands more from the implementation.
The UUID generator on TwoThink Dev only produces v4. Use v4 for ordinary business IDs; consider v7 only when you really depend on time ordering.