Are you sure it's only THIS season ?
I found info about the PCL and a bit of info on the IL.
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Went to a PCL game during the week, and found out about the league's unique DH rule:
1. If either club is an AL affiliate, the DH rules are mandated.
2. Otherwise the DH rules are optional with the concurrence of both teams.
As far as I know, there's no where else in pro baseball where the teams agree on the playing rules amongst themselves; usually all of that is mandated by the league.
There was nothing on the league website to indicate when the agreement for a given game has to be reached, eg, before the season or at gametime. Since there's 10 NL and 6 AL affililates, then the option situation comes up in about 60% of the home games for the NL affiliates, and obviously never comes up in an AL city.
By contrast, in the International League and at AA, there is no option; the DH rule is used unless both teams are NL, and then it is not used. In the IL, there's 6 NL and 8 AL affiliates, so the DH is in effect in about 40% of the NL affiliates' home games.