Why is datatype not defined on my custom field (circled below)? And how can I define it?
Those are actually standard fields. There appears to be a "glitch" or mistake by salesforce that's causing the datatype to be displayed in the Field Name column. It's the same in my dev org.
The Field Names values ought to be:
And the Datatype for these are
You can't define or edit the metadata of these standard fields.