This folder is the place where opinion modifiers are defined for the game. The default ones and focus tree ones are defined in 00_diplomatic.txt and 00_opinion_modifiers.txt.
An opinion modifier that is defined can be added to a country as followed :
SOV = { add_opinion_modifier = { target = GER modifier = ger_sov_alliance_focus } }
In the above example, that is part of the german focus tree, the Soviet union gains an opinion modifier of 50 called "ger_sov_alliance_focus" towards Germany. Within 00_opinion_modifiers.txt, this modifier is defined:
ger_sov_alliance_focus = { value = 50 }