What information might go in the remarks section of an IFR flight plan?
The remarks field, also called ITEM 18 field on ICAO flightplans can take a lot of information that you as the pilot deem necessary for controllers or authorities to know about your IFR flight.
The list of possible items in an ICAO flightplan ITEM 18 field is available over at the Skybrary, but here are a few examples that are commonly used:

(c) “TYP/” followed by type(s) of aircraft, proceeded if necessary by number(s) of aircraft, if “ZZZZ” is inserted in Item 9
Example: TYP/2 DE HAVILLAND HERON. Up to 60 characters may be used.
(e) “ALTN/” followed by the name of alternate aerodrome(s) or bearing and distance to navaid/navigation point closest to alternate point, if “ZZZZ” is inserted in Item 16. Up to 100 characters may be used.
Example: ALTN/MEDIUM AIRPORT
(k) “OPR/” followed by name of the operator, if not obvious from the aircraft identification in Item 7.
Example: OPR/BIG COMPANY
(o) “RMK/” followed by any other, plain language, remarks when required by the appropriate ATS authority or deemed necessary. There is no limit in number of characters to be used.
Example:RMK/TRAINING ILS APPROACH AT SMALL AIRPORT
RMK/STUDENT SOLO FLIGHT