For some ISPs, the IMPA/POP/SMTP username (and maybe other fields) has no relation to the email address and has to be entered separately. E.g. Email address is ben.bucksch@wong.com (chosen by user), but username is G675476 (provided by ISP).
In this case, an <inputfield> can be added for the username. It will let the client show a textfield to the user. The user-visible label is the one provided in the <inputfield label=""> attribute, to allow the ISP to use custom terminology for "username". The label is not localizable, that would be too complicated and most ISPs are local anyways- if really important, the ISP config server can look at the HTTP headers during the fetch.
The text that the user entered is written into a placeholder %key%, where key is the content of the <inputfield key=""> attribute. The placeholder can then be used in other settings, specifically the IMAP/POP and SMTP username.