What kinds of papers does the Indiana University Mathematics Journal publish?

The key portion of the FAQ reads:

The initial review is handled by the Managing Editor and by members of the Editorial Board and/or other departmental reviewers, depending on the area of expertise. This initial review is usually completed within weeks and, for most manuscripts, this is when it is determined whether or not a manuscript might eventually be accepted for publication. A paper deemed a candidate for publication will be sent to an external referee, whose report weighs heavily on the formal editorial acceptance or rejection of the manuscript.

The essential thing here is that the initial quick opinion and suggestion of a referee is done by members of the IU math department. So you should submit papers which are close enough to an IU faculty member's interests that they'll be familiar with the topic and able to suggest an appropriate referee. So all of mathematics is ok subject-wise (since we're a broad department), but within a given topic the closer it is to IU expertise the better the fit is.

The other two important parts in terms of your questions about turnaround is that the initial quick opinion step takes only a few weeks, and that there's typically a single referee which means it's faster than journals with two referees. Part of keeping the first stage fast is that sometimes papers are rejected simply because no appropriate referee was identified within two weeks, so please don't take it personally if this happens, the point is to keep the turnaround quick so you can submit elsewhere without losing time.

(Full disclosure: I am a member of the IU math department and do regularly review for them.)