An external identity will be resolved to the object

This will also be kind enough to try a few different alternatives to the externalid:

1. If you have a Guid called Guid (typed as Guid or string) property and it matches the sent in value we will resolve
2. If you sent in TheClassName;number we will take the internal class id from the name then try to resolve the result as externalid

See also The_ExternalId_explained