%SYSFUNC(function(argument(s))<, format>)
The second parameter to SYSFUNC() is a format, so whatever is processed as the first argument will get formatted as MONNAME9.
SYSFUNC
MONNAME format -> Writes date values as the name of the month.
MDY() function -> Returns a SAS date value from month, day, and year values.
SAS Functions by Category:
http://support.sas.com/documentation/cdl/en/lefunctionsref/69762/HTML/default/viewer.htm#p0w6napahk6x0an0z2dzozh2ouzm.htm
So first argument is creating a date and second formats it as a month.
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