F21 and C21 are both for the spouse of a US permanent resident. The difference between the two is that C21 is issued when the two people have been married for less than 2 years.
When someone is petitioned to immigrate as a spouse (i.e. either a US citizen or a US permanent resident directly petitions their spouse to immigrate), and the two people have been married for less than 2 years at the time the immigrant becomes a permanent resident, the immigrant becomes a conditional permanent resident, gets a 2-year green card instead of a 10-year card, and needs to apply for Removal of Conditions jointly with the spouse in the 90-day window before the 2-year card expires (the immigrant can do Removal of Conditions by themselves if there was divorce or abuse, etc.).
For someone doing Consular Processing abroad, they become a permanent resident when they enter the US with their immigrant visa. For someone doing Adjustment of Status in the US, they become a permanent resident when the Adjustment of Status is approved.
Note that, for someone doing Consular Processing abroad, it is possible for them to get the immigrant visa before their 2-year marriage anniversary, and get a C21 (or CR1 for spouses of citizens) immigrant visa, but as long as they enter the US with that visa after their 2-year marriage anniversary, they become a non-conditional permanent resident and will get an 10-year F21 (or IR1 for spouses of citizens) green card, and will not have to do Removal of Conditions. On the other hand, if they enter the US with the visa before their 2-year marriage anniversary, they will become a conditional permanent resident and will have to do Removal of Conditions. So what it says on the visa isn't really important; it's the timing of when they enter the US with the visa that really matters. See this chapter of the USCIS Adjudicator's Field Manual.
Note: It is extremely important that inspectors and adjudicators be
very conscious of the date of the marriage at the time the alien is
admitted or adjusted. It is not unusual for an alien to be issued a
conditional resident immigrant visa by a consular officer shortly
before the second anniversary, but to apply for admission after that
second anniversary. Likewise, an applicant for adjustment might file a
Form I-485 (or even be interviewed regarding such application) prior
to the second anniversary, but not be granted adjustment until after
that second anniversary. In such cases, the alien should be admitted,
or adjusted, without conditions (see 8 CFR 235.11(b) regarding the
authority of inspectors to amend the visa classification on an
immigrant visa in such situations).
Conditional permanent residents are almost always spouses of citizens, and it is rare to see them in the context of spouses of permanent residents, because spouses of permanent residents are in the F2A category, which has a wait for visa numbers of around 2 years anyway, so by the time the immigrant can immigrate, it will almost always be after their 2-year marriage anniversary. I am not sure about the timing in your case, but it could be that it is in a preliminary stage of the process, and it is before your 2-year marriage anniversary, but by the time your spouse gets the visa, it will probably be after the 2-year anniversary, in which case it will be an F21 visa. (But again, it doesn't really matter if your spouse gets an F21 or C21 visa -- what really matters is how long you've been married when they enter the US.)
