PARENT ID A
This is an Object ID that point directly to a Parent Object.
This represents a Direct Parent Relationship between one Object and another.
You might use it on a Contact record to hold the ID of the Company it belongs to. You could also use it on an Order Line record to hold the ID of the Order it belongs to.
Unlike a normal UDB relationship, It is only One to One. So you could only link a Contact to one Company.
It is however, significantly faster and more efficient than a normal UDB relationship.
See also CHILD ID X and CHILD ID Y.