I'm unable to add any picklist values to the Task field TaskSubtype. I actually can't even see the current values without looping through the describe results for TaskSubtype.
I have no problem accessing or querying the field, I've confirmed that I have customize application checked for my profile, the field is visible and editable to my profile, and it's listed as a field whose available picklist values are controlled by the record type. I saw in the Salesforce documentation that TaskSubtype is not a changeable field, but as far as I understand it just means the field is immutable. Am I wrong in that understanding, or am I just missing something else? Or is this just something you can't do?