Alternate keys is a useful functionality introduced from CRM 2016. It allows you specify one or more fields to be an alternate key and use it to identify a record instead of using GUID. This is extremely helpful when you work with data migration and/or data integration between systems. More about Altenate Keys here.
Only attributes of the following types can be included in alternate key definitions: Decimal, Whole Number, Single line of text.
However, there are some restrictions in what field can be selected as alternate keys which the document doesn’t mention. Below is the non-exhausted list of them, I’ll update this if I find more:
- Secured fields. A field with “Field Security” enabled doesn’t show up in the list when you creating alternate key.
- “fullname” in “contact” entity
- “domainname” in “systemuser” entity
So be aware when you work with alternate keys. Hope it helps!