ReaR creates a temporary directory using mktemp(1) and uses it among others for building its images, which can take quite a lot of space in some cases. It seems that in RHEL 9 /tmp started to be on tmpfs by default (although it could optionally be this way even in earlier versions, at least since RHEL 7 : https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/migration_planning_guide/chap-red_hat_enterprise_linux-migration_planning_guide-major_changes_and_migration_considerations#sect-Red_Hat_Enterprise_Linux-Migration_Planning_Guide-File_System_Layout-Temporary_storage_space ). Quite often tmpfs does not have enough capacity for ReaR and image creation fails. As the linked document says: "Processes that store a large amount of data, or require temporary data to persist across reboots, should use /var/tmp." And as file-hiearachy(7) says, 
"/tmp ... This directory (...) should hence not be used for larger files." "/var/tmp (...) can thus accept larger files. (...) If applications find the environment variable $TMPDIR set, they should prefer using the directory specified in it over directly referencing /var/tmp/".

So, the default for ReaR should be /var/tmp, but TMPDIR explicitly set by users should take precedence. I will implement it by setting TMPDIR to /var/tmp in /usr/share/rear/conf.default.conf if unset, as discussed under https://github.com/rear/rear/issues/2654#issuecomment-881609874