The same xasset program can also be used to query the current value of a global variable, or to deassign it (note that for XAS purposes a not existing variable, a variable with a null value, or a variable with all-blank value are equivalent : in all such cases most programs assume a suitable default value). Note however that if a non-XAS global variable with same name exists, its value may be used instead of the missing XAS variable.
This achieves a separation between the XAS and the system environment, by which system variables are readonly for XAS (they may be used instead of a missing XAS variable, but not set with xasset while XAS variables can be accessed via system facilities using their prefix).
The XAS environment is linked to a session, which in VMS means a process (and related subprocesses), while in Unix means a login session on a particular machine and a particular pseudo-terminal (in particular one must set the environment separately in each window !). To allow back-inheritance from a child process to the parent, in Unix the XAS environment is saved to a file : the date of creation of this file is compared with the date of login to determine whether it has still to be considered applicable.