Baltimore Celtic Union is a partnership between two clubs -- Baltimore Celtic and Baltimore Union. Whichever team has "Baltimore Celtic Union" in their name is the top team and has the best players from both clubs. Baltimore Celtic Union teams play GA in girls and ECNL in boys. The other two teams are the next strongest kids at each respective club.

2 different clubs. Union and Celtic have a joint venture type thing where they are supposed to feed their best players into the ECNL team.

https://www.soccerwire.com/news/collaborative-partnership-between-baltimore-celtic-union-to-include-boys-ecnl-teams/

That said, the union Pre-ECNL 07 team was Old Line FC last year - so not sure how many players that team fed into the ECNL team, or if the ECNL team is mainly legacy Union with a sprinkling of Celtic.

Can someone explain the difference between these boys teams?

Baltimore Union SC Pre- ECNL 07
Baltimore Celtic Union ECNL 07
Baltimore Celtic SC Pre-ECNL 07

Is there an A/B/C team system here? Do they share players? Does one feed into the other? Is it all one club? Thanks in advance