both 01 and 77 are exclusively for elementary items but 01 can also be used for non elementary items although 77 can't be used in case of a file but 01 can be used. so it depends but 77 is detected faster than 01
both these levels should begin in area a and must be followed by a seperator period;or by a space followed by its associated data name,filler or appropriate data description clause
I heard that 01 items will be stored in a ordered form , it can be easily fetched.
but 77 items will be stored where ever it find free space . so when we are using that 77 item , system have to search for the 77 item.