If you modify an issues issue's type by using the Move Issue Operation, in the changeitem table, the value of the field column will be set to "Issue Type". However, if you modify an issue's type by editing the system issue type field on a screen, or using bulk edit or something, the field column will be set to "issuetype".

Verified this with a clean instance reviewing the changeitem table after every change to an issue. I'm not sure if this actually affects anything, or if there is some reason why this is the case but it seems strange to me.