While editing dictionaries, you can add one or more identifiers in front of a word, after a "/". For example, the en_GB .AFF uses the identifier "S" to create plural:
party/S

This will look in the .AFF file and find:
SFX S Y 9
SFX S y ies [^aeiou]y
SFX S 0 s [aeiou]y
SFX S 0 es [sxz]
SFX S 0 es [cs]h
SFX S 0 s [^cs]h
SFX S 0 s [ae]u
SFX S 0 x [ae]u
SFX S 0 s [^ae]u
SFX S 0 s [^hsuxyz]

SFX S Y 9
SFX -> It is a suffix (PFX would mean a prefix).
S   -> The suffix identifier.
Y   -> Y for YES. It means the rule can be cross-used with other prefixes and suffixes.
       If N the rule can't be applied together with other affixes the word might have.
9   -> The number of lines related to this rule.

SFX S y ies [^aeiou]y
SFX       -> It is a suffix (PFX would mean a prefix).
S         -> It is the suffix/prefix identifier.
y         -> For a suffix it is the letter(s) to be removed from the end of the word.
             For a prefix, from the beginning of the word.
ies       -> For a suffix, it is the letter(s) to be added at the end of a word.
             For a prefix, from the beginning of the word.
[^aeiou]y -> Condition in regexp notation. Here, the rule is applied to words ending with
             a "y" and the letter next to the last is NOT a, e, i, o or u.
             Yes, the ^ means that the letters mustn't match.

So, party/S would produce: parties

And, boy/S would produce: boys, triggering the following rule which has a 0 saying that no letters are replaced, just added. It applies to words ending with a "y". There is no ^ which means that the second letter from the right must be a, e, i, o or u.
SFX S 0 s [aeiou]y

Also notice that if words have capitalised letters, the Hunspell in the used software will only accept them with capitalised letters exactly like in the .DIC (it suggests a typo if different).