The user keyring is associated with the record that the kernel maintains for the UID. It comes into existence upon the first attempt to access either the user keyring, the user-session-keyring(7), or the session-keyring(7). The keyring remains pinned in existence so long as there are processes running with that real UID or files opened by those processes remain open. (The keyring can also be pinned indefinitely by linking it into another keyring.)
Typically, the user keyring is created by pam_keyinit(8) when a user logs in.
The user keyring is not searched by default by request_key(2). When pam_keyinit(8) creates a session keyring, it adds to it a link to the user keyring so that the user keyring will be searched when the session keyring is.
A special serial number value, KEY_SPEC_USER_KEYRING, is defined that can be used in lieu of the actual serial number of the calling process's user keyring.
From the keyctl(1) utility, '@u' can be used instead of a numeric key ID in much the same way.
If it is necessary for a key associated with a user to exist beyond the UID record being garbage collected---for example, for use by a cron(8) script---then the persistent-keyring(7) should be used instead.