The letter designations used to indicate a maintenance interval inspection vary by the operator. Once an operator is established with the FAA as having an engineering and maintenance reliability program. It then has the authority to define its maintenance program and intervals as long as minimum criteria are met. This often results in various phases of maintenance that suit the operator's mode of operation. This even includes how maintenance is scheduled to be done. For example, America West's maintenance program is largely calendar based, while Alaska Airlines is keyed off flight hours and cycles. To answer your question, to be sure just what is meant by an "R" check, you have to know something about the particular operator and his program. Most of the checks you listed are typical designations for various levels of inspections.
The A check is a frequent inspection (often weekly) that can usually be accomplished in a single shift by a crew of mechanics. This check performs certain functional tests of instruments, exterior lights, filter changes, servicing landing gear, checking cabin emergency equipment, etc.
The B check is an intermediate frequency inspection (often monthly) that is more comprehensive than the A check, but does not require significant disassembly. More systems are tested and some stuctural inspections may be incorporated. This check can typically be accomplished in 1-2 shifts.
The C check is a heavy maintenance visit that occurs roughly every 3,000 flight hours (remember this varies by operator). Depending on the operator's program, this may take from 5-30 days to accomplish. It is common for operators to tailor their program to take advantage of this down time by scheduling other inspections concurrently, so portions of the SID (structural inspections) along with the C check.
The D check is essentially an overhaul. These inspections generally incorporate most, or all, of lower level inspections and all the structural inspections. This type check may require up to 75 days, depending on the aircraft and the intensity of the maintenance requirements. Not all operators have a separate D check, but rather segment the items in a D check and accomplish these items incrementally during the C checks. When a cycle of C checks is complete, the requirements of the D check have been met.
The other letter designations you listed need to be tied to the operator to be sure of their scope of work.