TXM variables are the attributes of a BIM, workstation, PDS, or VSE file. "Last referenced file" is another way of saying "the file currently indicated by the TXM variables". The attributes of the file are used to initialize many of the SSP variables when an EDIT or LIST session is started.
Commands which access files, such as FileAlter, FileBrowse, FileEdit, FileList, or FileRead, use and set the TXM variables. There is one set of TXM variables, which are initialized at startup and held until shutdown. The values of the TXM variables remain constant until changed by a command that accesses a file by name or a command such as SessionClose or SessionRotate, which accesses a file implicitly.