Outfit question

Malory

So I'm a ftm and have taken care of and dressed many babies however I never had to worry about the size of the outfits I'm so confused why are some 0/3 and some 0/6 is that saying it should fit up to 6months?? I know I should have a few different sizes for baby to come home in but it just doesn't make sense and I'm not sure what clothes I should have stocked up for him. Please help lol