Hi Rishika Kummithi,
Both ways to type KUNNR and KNA1-KUNNR are correct. In both situation you are declaring an element from a specific STRUCTURE.
When you have several elements in your pool like KUNNR and KNA1-KUNNR, to make a distinction between those elements, you can declare the structure table where it came from (KNA1-KUNNR).
This scenario is very frequent when you work with custom solutions, the custom elements can have the same technical name but different types like (DATUM and CHAR10). If you specify in the structure, witch element you are choosing with the table, you can debbug any incident with more precision.
I hope this help you,