Happy Wednesday @StephanieC and thank you for reaching out!
Great question! As it stands, the sum feature is only available for numerical fields. And these type of fields are currently not included when setting a Rule.
We have a thread in the #productfeedback category regarding this topic. In case you haven’t yet, I encourage you to cast your vote in this request: Rules set Custom Fields with Text or Numbers - #2 by Natalia
In the meantime, I would suggest you to have a look at the Integration Flowsana who allows you to use all types of Custom Fields when setting a Rule.
I hope this helps Stephanie! Please let me know if you have any follow-up question!