I'm writing documentation for some code at work and I am trying to describe some parameters that must be specified by the user. Some of the parameters are specified with one word from a list of possible choices (e.g., "true" or "false"; "ags" or "wms"; "json", "xml", or "text"). Others are more "open-ended" and the user specifies anything they'd like (parameters like this include name, identifier, URL, etc).
I'd like to describe each of these two types of parameters. Are there words that describe one as open-ended and one as multiple choice? These descriptions I'm using don't seem to do it justice. They also seem to imply there is some kind of question involved when there isn't.
Does anyone have any ideas of what words I could use in my documentation to describe these two types of parameters?