I have more logon.src in more packages.
One logon.src should start as first and shound disable all logon.src to start.

A "masterlogon.src", unique in pol script, that can return 1 to allow all logon.src to start or return 0 to not allow all logon.src to start?

Scripts/misc/logon.src <-- main logon.

In there, you can set a CProp on the character, that could be to block the others, or allow only certain ones, based on that CProp. Then, in the other logon.src scripts, you can check that CProp at the very beginning, to decide if you want that one to run or not.

The other logon.src start only after the main is finished?

Yes because
1) as you can see here, the default priority of logon.ecl scripts is critical
2) the main logon is the first one run by the core
the two things together give you the answer

In Laymon terms for lagoon : RTFM

Gosh darnit, now who would read the manual, and put together information from it to draw logical conclusions?

Next thing you know, you all will be expecting us to write our own scripts!

In manual is written yes that is critical but not that logon.src in misc start for first and I like to be sure before do things.

A good approach .. how many of us learned, long before the docs were put up... experiment, lots. Trial and error. Then you know for sure.