Cinema 4D versions come in two different licensing options, Subscription (S) and Full Release (R)
If you have an active subscription licence, you are permitted to use both the S and R versions.
If you have a perpetual licence, you are permitted to use up to the release you own.
An R21 Perpetual user can use R21 only.
An R23 Perpetual user, can use R23, S22 and R21 - they cannot use S24 or onward, as this is a subscription-only release.
An R25 Perpetual user can use, R25, S24, R23, S22 and R21, but cannot use S26.
A subscription customer will be able to use the latest version, either S or R.
Cinema 4D 2023 removed the S/R and is a subscription only product.