effect_type determines both what comparisons look at and what the effect is applied to. Throw in target_self and the comparisons will still look at the category after effect_type but the effect will apply to self.

Example:

{
"effect_type": "adjacent_symbols",
"comparisons": [{"a": "type", "b": "cat"}],
"target_self": true,
"value_to_change": "value_multiplier", "diff": 2
}

The example symbol will make itself give 2x as many coins if it is adjacent to a cat.