English Hyphenation file looks something like this...
LEFTHYPHENMIN 2
RIGHTHYPHENMIN 3
COMPOUNDLEFTHYPHENMIN 2
COMPOUNDRIGHTHYPHENMIN 3
.a2ch4
.ad1d4
.am5at
1, 2, 4 and 5 numbers are added next to characters. Does it mean it is OK to hyphenate next to "a" but not OK to hyphenate next to "ad"?
You should apply all rules to your word (those that apply). Larger numbers prevail over smaller ones if they are in the same place. Odd numbers define where hyphenation should occur.
(Partial) example with the word 'aviary'
av i ary
av1i <- rule 1
v2i1a <- rule 2
av2i1a <- compound rule (2 wins over 1)
The final hyphenation (with the rules above) should be avi-ary.
See https://github.com/hunspell/hyphen/blob/master/README.compound