What are the rules when setting default values for AGOL hosted feature layer fields? (Admin REST API)

[Bud]

What are the rules when setting default values for AGOL hosted feature layer fields? (using the Admin REST API)

For example:

1. Use SQL Server's SQL syntax/dialect.
   1. GIS Stack Exchange: What kind of database does ArcGIS Online use to store data?
2. GUID and number fields support expressions, but text fields don't.
   1. Idea: Allow expressions in AGOL string field > default value
3. Here's a useful error message I got when trying to use OBJECTID+1 as the default value for an integer field:
   "Valid expressions are constants, constant expressions, and (in some contexts) variables. Column names are not permitted." Link
4. Are there any other known rules? Or any other useful error messages that could be noted here?

(To Esri staff: Could the rules be added to the AGOL docs?)