ALV

I am creatin a field catalog by using func module REUSE_ALV_FIELDCATALOG_MERGE. In this we have to pass the Internal Table name. IT_DATA is the name of my internal table. When I declare it as

IT_DATA TYPE IT_TYPE OCCURS 0 WITH HEADER LINE.

its gives a runtime error that 'Field catalog not found'.But when I declare my internal table by using

BEGIN OF IT_DATA OCCURS 0,

....

....

END OF IT_DATA.

It is working.

Please tell me the difference in these two declarations.