The APEX_DEBUG package uses the following constants.

subtype t_log_level is pls_integer;
c_log_level_error constant t_log_level := 1;
    -- critical error
c_log_level_warn constant t_log_level := 2;
    -- less critical error
c_log_level_info constant t_log_level := 4;
    -- default level if debugging is enabled
    -- (for example, used by apex_application.debug)
c_log_level_app_enter constant t_log_level := 5;
    -- application: messages when procedures/functions are entered
c_log_level_app_trace constant t_log_level := 6;
    -- application: other messages within procedures/functions
c_log_level_engine_enter constant t_log_level := 8;
    -- APEX engine: messages when procedures/functions are entered
c_log_level_engine_trace constant t_log_level := 9;
    -- APEX engine: other messages within procedures/functions