Uniq.UUID.uuid4

You're seeing just the function uuid4, go back to Uniq.UUID module for more information.
Link to this function

uuid4(format \\ :default)

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Specs

uuid4(format()) :: t()

Generates a UUID using the version 4 scheme, as described in RFC 4122

This scheme is like the version 1 scheme, except it uses randomly generated data for the timestamp, clock sequence, and node fields.

This scheme is the closest you can get to truly unique identifiers, as they are based on truly random (or pseudo-random) data, so the chances of generating the same UUID twice is astronomically small.

Notes

The version 4 scheme does have some deficiencies. Namely, since they are based on random data, the lexicographical ordering of the resulting UUID is itself random, which can play havoc with database indices should you choose to use UUIDs for primary keys.

It is strongly recommended to consider the version 6 scheme instead. They are almost the same as a version 1 UUID, but with improved semantics that combine some of the beneficial traits of version 4 UUIDs without the lexicographical ordering downsides. The only caveat to that recommendation is if you need to pass them through a system that inspects the UUID encoding itself and doesn't have preliminary support for version 6.