Ok so h2h has to be able to determine all the seeds for the tied teams. In order to do so, all teams must have played each other in that tied record scenario. If not, you obviously can't determine a seeding with h2h. After all teams have played each other, the following must be true.
Team A must have beaten all teams, Team B must have beaten all teams except A, Team C must have beaten all teams except A and B, Team D must have beated all teams except A, B and C. And it goes on.
Let's say we have a tied scenario of 3 teams.
Team A has beaten B and C.
Team B has beaten C and lost to A.
Team C has lost both matches.
All teams have played each other and the other condition is true. We can use h2h here.
Let's look at another scenario:
Team A has beaten B, C and lost to D.
Team B has beaten C, D and lost to A.
Team C has beated D and lost to C and D.
Team D has beaten A but lost to B, C.
Obviously here it's inconclusive. Even though A has beaten B and C they have lost to D which means you have a sort of circle rather than a clear order. In this case, we go to Rounds For.
I could do another scenario with one team not playing the others but I think you get the point.
Here is another rare scenario that sometimes happens:
RF RA
A 23 22
B 21 22
C 21 20
Let's say
A has beaten B but lost to C
B has beaten C but lost to A
C has beaten A but lost to C
Obviously we go to RF since h2h is inconclusive. A is clearly ahead in RF so they advance as 1st seed but we still have to break the tie between B and C. Some people think that since B has beaten C they should be the higher seed but that is incorrect. You don't go backwards in terms of tiebreaking, you always continue on. Even though B has beaten C, C has less RA so they are awarded seed 2.