$X contains an integer value ranging from 0 to 65,535, specifying the horizontal position of a virtual cursor in the current output record. $X=0 represents the left-most position of a record or row.
Every OPEN device has a $X. However, M only accesses $X of the current device. Therefore, exercise care in sequencing USE commands and references to $X.
Generally, GT.M increments $X for every character written to and read from the current device. Usually, the increment is 1, but for a process in UTF-8 mode, the increment is the number of glyphs or codepoints (depends on the type of device). M format control characters, write filtering, and the device WIDTH also have an effect on $X.
$X never equals or exceeds the value of the device WIDTH. Whenever it reaches the value equal to the device WIDTH, it gets reset to zero (0).
GT.M follows the MDC Type A recommendation and permits an M routine to SET $X. However, SET $X does not automatically issue device commands or escape sequences to reposition the physical cursor.
For more information, refer to “$X”.