Customer Tables - SU25

Hello Experts,

As of my understanding, the data what I am referring in XD01 is the Customer Master table KNB1.

But in Tcode SU25, Step 3, We have a text like "  Import Customer tables".

I understand very well that the Customer tables in SU25 refers to USOBX_C and USOBT_C.

Since the table KNB1 called as Customer Master Table, why tables USOBX_C and USOBT_C are also referred as Customer tables?

Thanks in advance.

Regards

Manoj