what does preference classification: 201 B INA spouse of USC mean?

[TJ 4EVER 40]

Was looking at my NOA1 and was just curious what 201 B INA spouse of USC mean. I know spouse of USC mean but the latter code I don't understand. Just curious. Also tried to check status with receipt number and it is not working.

[NigeriaorBust 4,087]

Immediate Relatives

§201(b) of the Immigration and Nationality Act ("INA") defines "immediate relatives" to include:

1. spouses,

2. minor children (under the age of twenty-one), and

3. parents of U.S. citizens (if the citizens are at least 21 years old).

[TJ 4EVER 40]

thank you