I have tested RM's Description field before, but I just now ran a new set of tests.
RM's Description field is exported to GEDCOM in its entirety, no matter how long it is. RM does not use any kind of CONT mechanism for the Description field. Rather, it violates the GEDCOM limit on 255 characters per line and puts out the whole thing in one GEDCOM record no matter how long that GEDCOM record is.
When RM's Description field is imported, if the Description field is longer than 100 characters, it is truncated to no more than 255 characters and is placed in the note field for the same fact. Just to be very clear, the placing of the Description field into the Note field does not take place at 100 characters or less, and it begins happening at 101 characters. As long as the Description field is 100 characters or less, it is imported back into the Description field. Beyond the first 100 characters, no data is lost unless the length of the imported Description field exceeds 255 characters, and those characters in the Description field after the first 255 are lost.
If the same fact already contains a note when the Description field is moved to the Note field, the Description field is placed at the front of the existing note and is separated from the existing note by a carriage return / line feed sequence.
If the Description field for a fact in RM contains any trailing blanks, the trailing blanks are trimmed on export. The trailing blanks are not trimmed during data entry and they can be seen in RM's database via SQLite. But because the trailing blanks are trimmed on export, they are not seen in an import and therefore are not an issue on import.
I make heavy use of RM's Description field and I always keep it at 100 characters or less. I like the fact that it's available as a column in People View or as a column in Custom Reports. I like the fact that it's searchable and can be used for color coding and in the creation of Named Groups. But from this thread, it sounds like RM's Description field violates GEDCOM standards for some fact types. Therefore, I wonder if I should revisit how I use it.