Abintio Record Format- Setting NULL to variable

I have a question regarding Ab Initio CoOperating system.

I have a compoents input record format with a field set to NULL, like,

record
string(3) INPUT = NULL;
end;

Comp A -----FlowAtoB---------> Comp B

The above record format is the FlowAtoB. I can understand that this sets the value of INPUT to null. But, In case I have value for this from previous component A, how will this behave?

<a href= https://tekslate.com/ab-initio...>Ab Initio</a>

I'm not able to clearly understand the use of this from the help document.