DIR_TRANS is a SAP Parameter attribute which is used to control root directory for transports information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
A system group is delivered centrally with imports from this directory
or all exports are collected on this directory that is mounted globally.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.