Thanks for all help, Grayfort and Lightfoot8.
Let's see the problem in a practical way, so I can learn to solve and understand it better...
I have to take my custom 2da (the mainly 2da) and check every new entry in the merging process to place in any empty line all 2.4 cep stuff from other 2da, to keep safely my custom classes, custom tilesets and other stuff, protected against accidental overwriting?
In other worlds, I take my custom 2da to use as "main" file and overwrite the cep 2da over, taking care to not overwrite my custom stuff and choosing any empty line to place the new stuff and skip the which is equal to my custom 2da or other stuff that I do not want to copy or upgrade?
If yes, I have to find the right tool to compare each line in the 2da file, move and copy whatever is necessary. Anyone said notepad?
Now I have some questions...
1 - I need to follow any special rule to place some kind of stuff in the right place to avoid bugs? Like "you can use only line 12 to 25 to place tilesets, line 50 to 70 to place feats info" and other things like this?
2 - Using Cep (and any community custom content) I have to follow any certain rules to keep the stuff in the right place for the other players? In a example, if I place a plc in my custom 2da in a 325 line, but if the CEP 2.4 use the same line to show a dagger, what works right? The cep hak or my line, because my module and the custom 2da works to redirect the stuff using my own haks (and the cep haks in a lower priority, in my 2da)?