Can I use autonumber fields as External Id?
Yes. You can use autonumber fields as an External Id, but for Insert operation only.
- For other operations which might include record update, autonumber fields CAN'T be used as External Id, since autonumber values are generated independently in each org, so they will never overlap between the Source and the Target orgs.
- If you use autonumber for insertions, actually that does not matter, because the Plugin still internally replaces your autonumber with Record Id field when performing Insert operation.