What is the semantic meaning of the RouterCallKeyDay variable?

What is the semantic meaning of the RouterCallKeyDay variable?
A . It represents a number that corresponds to the day that the call was taken. For example: at midnight it could increment from 151191 to 151192.
B . It represents a string that corresponds to the day that the call was taken. For example: at midnight it could advance from "Monday" to "Tuesday".
C . It represents a number that uniquely identifies the call during the day it was taken. For example: at midnight it would reset to zero.
D . It represents a sequence number used for ordering rows for the same call.
E . It represents a string that corresponds to a Globally Unique Call Identifier.

Answer: A