belated return question

kinnary (student) (231 Points)

10 February 2009  

 Late Return/Belated Return

Any person who has not filed the return within the time allowed u/s 139(1) or u/s 142(1) may file a belated return at any time before

...the expiry of one year from the end of the relevant assessment year or

...before the completion of the assessment, which ever is earlier.

so my question is what does it mean by "before the completion of the assessment".when return itself is not filed then how can we say that belated return can be filed before the completion of the will assessment be done without filing the return.