APEX debug levels

From time to time I run into some new stuff about APEX. So few days ago I was investigating something about debug messages and figured it out that there are different levels of debug messages in APEX.

You probably know that as a sixth argument in APEX URL you can pass either YES or NO and that will turn on/off page debugging (if it's enabled in application properties). So the thing I discovered is that you can also pass LEVELn where n is between 1 and 9 and stands for level of debug detail to display. The value of YES is equal to LEVEL4.

If you take a look at apex_debug package specification you can see meaning of each level (although missing levels 3 and 7):

-- critical error
c_log_level_error        constant t_log_level := 1; 

-- less critical error
c_log_level_warn         constant t_log_level := 2; 

-- default level if debugging is enabled (e.g. used by apex_application.debug)
c_log_level_info         constant t_log_level := 4; 

-- application: messages when procedures/functions are entered
c_log_level_app_enter    constant t_log_level := 5; 

-- application: other messages within procedures/functions
c_log_level_app_trace    constant t_log_level := 6; 

-- apex engine: messages when procedures/functions are entered
c_log_level_engine_enter constant t_log_level := 8; 

-- apex engine: other messages within procedures/functions
c_log_level_engine_trace constant t_log_level := 9; 

You can set this level when adding debug messages manually using apex_debug.message procedure.