RAM buffer

In addition to bandwidth and capacity variants, modules can
1. Optionally implement ECC, which is an extra data byte lane used for correcting minor errors and detecting major errors for better reliability. Modules with ECC are identified by an additional ECC in their designation.
2. Be "registered", which improves signal integrity (and hence potentially clock speed and physical slot capacity) by electrically buffering the signals at a cost of an extra clock of increased latency. Those modules are identified by an additional R in their designation, whereas non-registered (a.k.a. "unbuffered") RAM may be identified by an additional U in the designation.
3. Be Fully buffered modules, which are designated by F or FB and do not have the same notch position as other classes. Fully buffered modules cannot be used with motherboards that are made for registered modules, and the different notch position physically prevents their insertion.