The labels "Pre-xxx" were used by clubs that played in the ECNL to keep consistency in the way they named/labeled thier teams. ECNL did not have leagues for the youngers (below U13). The older teams were named by year group and league, ie 2010 ECNL (for the A team), 2010 ECRL (for the B team), 2010 Socal Flight 1 (for the C team). Because the U12 and youngers did not play in ECNL the youngers A teams would be labeled 2016 Socal Flight 1 (or whatever league they played in).
This created some confusion with parents of youngers, who do not understand how the leagues and team ranking work, about which team was the A team. To help make things clear to the Youngers, the U12 and U11 "A" teams were usually named something like 2016 Pre-ECNL. Indicating that when this team aged up to U13, it would in fact be playing in the ECNL, because the clubs older teams were currently playing in the ECNL.
Then things started getting stupid. Clubs that did not have olders playing in the ECNL, but were trying to get accepted into the ECNL started naming thier Older Teams, Pre-ECNL (or Pre-EA, or Pre-xyz, this happened across the board with all the leagues) as a marketing strategy to get people to believe they were going to be joining the ECNL.
To create further confusion, the ECNL then created an actual league named Pre-ECNL were the ECNL Clubs could play thier U12/U11 Younger Teams against other ECNL clubs. So there is actually a Pre-ECNL league that some Pre-ECNL teams play in, there are some ECNL Clubs who name thier younger "A" teams Pre-ECNL for continuity, and there are clubs who name thier teams Pre-ECNL who don't actually play in the ECNL. This occurs with all the Letter Leagues.
Bottom line: if the Clubs older teams are actually playing in the league, the Pre-xyz indicates which team will move into that slot at U13. If the Clubs older teams are not playing in the league, it is a major red flag as the club does not understand the soccer landscape, or they do and thier intention is to mislead you.
As a side note, NPL is team based promotion and relegation based on previous season performance. There is absolutly no garrantee that a club with older NPL teams will have the U13 team accepted automatically into the NPL league like ECNL, GA, MLS,etc.. The Pre-NPL team designation is a BS assertion becuase maybe they will and maybe they won't.