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/usr/lib/xymon/client/local/README is in xymon-client 4.3.28-3build1.

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This directory - the client/local/ directory - can be used to
install Xymon client add-on scripts. The Xymon client will run
all files in this directory that are executable, and include the
output from each script in a separate section in the Xymon client
message which is sent to the Xymon server.

This output will have to be processed on the Xymon server; there
is no default processing done by Xymon on the output from these
scripts. They are merely added to the client data.

If you want to install an add-on script that direcly generates a 
status column in Xymon, this should go in the client/ext/ directory
instead.