so, wiki template syntax supports something called #switch.
the switch can have a "default", or not. when a parameter is _only_ used in a switch, and there is no "default", it means that only one of the suggested values will have any effect, and everything else is equivalent to no value at all.
the TD syntax should allow distinguishing between those two types of use. currently, the help page says
"If the user needs a value not included in the list (for example, "message in a bottle") they can type it in manually."
this makes sense for some parameters, and no sense for others (i.e., params only used in #switch with no default).
note that the TD is used by things other than the visual editor, for which this feature was requested for the longest time - see, e.g., phab:T224342.
peace