From the manual
BIN, DEC and HEX may also be followed by a number. Normally, these modifiers display exactly as many digits as are necessary, zero blanked (leading zeros are not sent). However, if a number follows the modifier, SEROUT2 will always send that number of digits, adding leading zeros as necessary. It will also trim of any extra high order digits. For example, BIN6 8 would send A001000" and BIN2 8 would send A00".