it is an after logon trigger. It is an online patching related thing.
You can always view its code using;
SQL> show long;
SQL> set long 20000
SQL> select dbms_metadata.get_ddl('TRIGGER','EBS_LOGON','SYSTEM') from dual;
It gets the service name that you are connecting to using sys_context userenv.
If you connect to the patch env. I mean if you are connecting to the database using patch service name, then the trigger checks if there is a E-Business Suite Patch Edition exist in the database. If not it gives error.
it reads the service name that you are connection to and the db_domain from your session env.
IF the service name is ebs_patch;
it gets the patch edition name
it sets the current edtition to patch edition.
it alters your session so that pathing actions will wait for locks if needed
It gives error if E-Business Suite Patch Edition does not exist