Thanks very much @ahoward, that information is helpful.
I've now managed to create Camera and Film fields by repurposing existing fields. While doing this, I discovered that the existing fields don't all work the same. One accepted more than one value from the .TXT file that I created for its drop-down menu; the other didn't, simply replacing a value that I selected from the drop-down menu with the second value that I selected. Good to be aware of this.
I'm finding that I prefer taking the time to write metadata .TXT files in TextEdit, and import them, to writing data values in the Photo Mechanic window. I feel that it helps me better understand how metadata input works.