Then you do want it to be semi-permanent - until such time as it needs to be changed. So many ways you could do this.
A table for db parameters is one way to go. I sometimes use these for various aspects of a database. It's not likely that you can include such a field in your form's underlying record source, and if you show the value via DLookup, you can't edit such a calculated control if you want to change it. You'd need an alternate method such as a form for editing db parameters or a popup form just for this one value.
If the values are a static list, then you could use a combo on said form with either a value list or again, a table of records. This way a different value can be chosen from a set list. Being unbound, you'd have to set the target control default property on AfterUpdate of the combo.
Anything else I can think of would need to be input every time the form opens, which isn't what you want, I guess. You don't want anything like option buttons that would require modifying form design like that. Truthfully, altering a value list falls into that realm AFAIC. Likely others will chime in with good or even better ideas.