So kinda cool but unexpected, invested in cap collector to increase my merchant cap supply to make unloading water farm easier.
But turns out 1 investment per type of merchant covers every single store over every single settlement. Thought I had to invest in each store, or at least each type of store per settlement, but I have 10+ settlements, each with 10+ stores (1 copy of each kind, plus double of each happiness store per settlement)
500 cap investment to my general trader in sanctuary increased cap supply with every single general trader over all settlements. Same for each other store type.
Haven't invested yet in NPC merchants like at Diamond City, I'm hoping that one is more a 1-to-1 basis of investment. Would be weird to invest in 1 DC merchant and have every single store of that kind in other places also increase cap supply.