On 18 Oct 2022 at 07:18p, Clive Reuben pondered and said...
 
 CR> -------->> start concerning entries <<------------
 CR> 2022.10.18 13:36:06 Executing: sh /mystic/temp2/xfer.sh
 CR> 2022.10.18 13:36:06 Execution complete: 32512
 CR> -------->> end concerning entries <<--------------

Here we go, found it. It's mentioned in whatsnew.txt from back in the development of 1.08

[snip]

+ Added the ability to execute an MPL program instead of the command line
   for a protocol.  By starting your command line with a !, you can have
   Mystic run a MPE program.  For example:

       send Command: !test %1 %2 %3

   The above would execute test.mpe from your scripts directory and pass
   the results of the %1 %2 %3 protocol MCI codes as command parameters to
   the MPL program.  Keep in mind that if you do use this to execute some
   type of protocol, you must set the DSZLOG environment variable yourself
   and have it point to the current node's temp directory as xfer.log.  Mystic
   will also create an xfer.bat or xfer.sh (depending on operating system) which
   can also be executed.

[snip]

Kerr Avon [Blake's 7] 'I'm not expendable, I'm not stupid and I'm not going'
avon[at]bbs.nz | bbs.nz | fsxnet.nz 

--- Mystic BBS v1.12 A47 2021/12/24 (Linux/64)