> The place for small temporary files. This directory is usually mounted as a tmpfs instance, and should hence not be used for larger files. (Use /var/tmp/ for larger files.) This directory is usually flushed at boot-up. Also, files that are not accessed within a certain time may be automatically deleted.
Source: https://uapi-group.org/specifications/specs/linux_file_syste...
/tmp is for stuff that is 'absolutely' temporary, in that on many/most systems it is nuked between reboots. /var/tmp is 'relatively' temporary in that applications can put stuff there that they're working on, but if there is a crash, the contents are not deleted and can be recovered across reboots.