BAP and Roster Question for the forum

I almost exclusively play BAP (goalie). I usually buy the game and go thru the Europe tourney and get drafted and play the game. It’s pretty simple. 

But this time I tried something different. I wanted the rosters to be updated before I started a BAP career. I waited until the Oct 15 roster update to start a new career. I did not edit any players. So I have started a new career with the 1st roster update and I see that there is a Oct 20 roster update but my game is currently using Custom roster Oct 20. So Im assuming that the BAP game made it for my current BAP. 

So question is.. What does “merger rosters” do? Does it matter for BAP or is that for other modes? If I merge the Custom Roster with the current online Roster, what would that do?