Out of curiosity I've tested it (as I have never before paid that much attention to it) and it doesn't seem like a superfluous character like @KenFarley suspected. From what I saw it looks like this:
1. ENV=VARIABLE_NAME=VALUE
will set the variable VARIABLE_NAME with only a VALUE defined in psf file, e.g. ENV=PTC_D_LICENSE_FILE=7788@license_server will get you variable PTC_D_LICENSE_FILE=7788@license_server regardless of its previous value.
2. ENV=VARIABLE_NAME+=VALUE
will append the value at the end of the VARIABLE_NAME existing values list. e.g. ENV=PTC_D_LICENSE_FILE+=7788@localhost, will get you PTC_D_LICENSE_FILE=7788@license_server;7788@localhost (appending to the previously defined variable value of 7788@license_server)
3. ENV=VARIABLE_NAME-=VALUE
will place the new value in the beginning of values list for that variable, e.g. ENV=PTC_D_LICENSE_FILE-=7788@localhost will get you PTC_D_LICENSE_FILE=7788@localhost;7788@license_server (so when Creo starts your local license server will take precedence when searching for Creo licenses and when it's not available Creo will fall back to the next server in line).
Hope it makes sense. You can see that PATH variables are defined in similar way, using "+" and "-" to not overwrite existing PATH values but to append the new values at correct places.
