What exactly does merge during icloud setup mean? What is merged if I did merge? I’m aware about contacts.

I was signing into a different apple ID for my ipad and I wanted to keep just contacts and such on there so I clicked merge but I wasn’t aware that it would merge contacts from that other apple ID onto my phone and basically within that apple ID I thought it was just going to keep certain things on the iPad. It’s not a huge deal as the other apple ID is with a family member but I just want to make sure that things like photos and notes didn’t merge.