Question: Where is the help text for this environment variable and others like it?

In a Maple Primes reply by Joel Riel on Sept. 14, 2011 he included the following command referring to warnings issued from a dsolve, numeric command having events containing a halt action:

_Env_in_maplet := true:  # incantation to suppress integrator warnings

Where can if find explanatory help for his command and any others of a similar nature?
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