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I am trying to make a procedure return a subset of records as a single type a_ntt meeting a certain requirement for which I am trying to define a cursor a_cur , and this is the relevant code for my question:

a_t a_cur%ROWTYPE;
TYPE a_ntt IS TABLE OF a_t; 
l_a a_ntt;
My database server throws PLS-00488 (invalid variable declaration object must be a type or subtype) considering variable a_t. I would like to know why. To me it seems I have provided a type to a_t, I would suggest  ROWTYPE does that for me, but obviously I am not getting something. 
I would like to know why this error occurs, why declaring a_t as a_cur%ROWTYPE does not prevent this.
Just to make sure, I have resolved the issue by adding %TYPE to the line, so TYPE a_ntt IS TABLE OF a_t%TYPE;, but I would like to know why this is necessary.
a_t is not a type. It is a variable whose type is a_cur%ROWTYPE. You can't use it as a type.
a_t%TYPE is a type though, so that works.
The syntax diagrams for the "collection variable declaration" is in the PL/SQL docs. You'll see that either you need a type, or a rowtype_attribute which has cursor, table or view name with %ROWTYPE attached, or a type_attribute which is essentially a variable name with %TYPE attached.
        
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