The age group comes first and it's predominantly done in school years. the number is the maximum age of the rower in that year so someone may be 14 yet rowing as a J15 as they are in year 10.
J14 - year 9
J15 - year 10
J16 - year 11
J17 - lower sixth
J18 - upper sixth
Most of the time they will merge lower and upper sixth together under the category J which stands for Junior. Anyone can row in a category above them (i.e. a J14 rowing as J16) but you're not allowed to go down a year.
If entered at adult level a points system occurs making things harder. One win at adult level (not a J category) and it is one point. 1 pot, 1 point.
N - Novice - you cannot enter with 1 point
IM3 - Intermediate 3 - you cannot enter with 3 points
IM2 - Intermediate 2 - you cannot enter with 5 points
IM2 - Intermediate 1 - you cannot enter with 7 points
S - Senior - you cannot enter with 10 points
E - Elite - no limit
Here's a summery of the boat types and their respective symbols:
1x | 2x | 2- | 4x+ | 4x(-) | 4- | 4+ | 8+ | 8x |
Single | Double | Pair | Coxed quad | Coxless quad | Coxless four | Coxed four | Eight (always coxed) | Octo (always coxed) |
I hope this helps with understand what boats have been entered