8老師，485這麽說的，有點不清楚

Family-based preference categories include: unmarried sons and daughters (21 years of age and older) of U.S. citizens; spouses, unmarried children (under 21 years of age) and unmarried sons and daughters (21 years of age and older) of lawful permanent residents; married sons and daughters of U.S. citizens; and brothers and sisters of U.S. citizens (if the U.S. citizen is 21 years of age or older).
If a visa is immediately available, applicants filing under a family-based preference immigrant category do not have to wait until Form I-130 is approved to file Form I-485. If a visa is immediately available, you may file your Form I-485 together with your Form I-130, while Form I-130 is pending, or after your Form I-130 is approved. Otherwise, you may file your Form I-485 only after your Form I-130 is approved and a visa is immediately available. See the When Should I File Form I-485 section for more information.

前麵說可以不用等到130批，後麵又說otherwise , 必須130批並且簽證有名額，我的是F2B. 這裏到底是什麽意思？