Here is a list of all concepts with brief descriptions:

Nwowlib
Ndb
RTableRecord	A type the table engine can carry as its record: a flat generated (or hand-written test) struct naming its client version and table
Nformats
Ndetail
RHasConvertStep
RHasValidateExtra	A type carrying the imperative half of its contracts (see the validateExtra hook)
RSlotSequence	A container of independent SLOTS rather than one logical array — a chunk that may appear several times, each occurrence complete in itself (Repeated<>, opting in with ValidationSlots)
RValidatableSequence	A member the walker can iterate element-wise: std::vector, std::array, Repeated<> and std::string all qualify
Nm2
ROffsetStringMember	A member serialized as an M2Array<char>: its bytes (NUL included) live in a data block, referenced by an M2Array{count, offset} slot
ROffsetArrayMember	A member serialized as an M2Array{count, offset} reference to a separate data block — a std::vector (the block holds its element images) or a std::string
RInlineRecordMember	A member serialized inline as a nested record: a non-trivial class the walker recurses into member-by-member at the current cursor (M2Track and friends)
RInlineScalarMember	A member serialized as inline raw bytes: anything trivially copyable that is not an array/string reference (scalars, C3Vector, M2Range, …)
ROffsetEntity	A type the offset engine can read and write: it is an M2 offset block and declares the client version it is laid out for
RChunkedEntity	A type the chunk serializer can read and write: carries the round-trip bookkeeping and knows which client version it is laid out for
RSelfSerializing	A member type that owns its chunk-payload encoding: the serializer hands it the raw payload on read and the output buffer on write (StringBlock, ChunkBlob)
RVersionedEntity	An entity the walker can gate by client version: anything carrying the static constexpr ClientVersion version every format entity declares (chunked files, M2 offset blocks, ADT tiles and map chunks alike)
Nfs
RListfileProvider	The pluggable path<->FileDataID database contract (CSV today, SQL later)
RStorageBackend	The storage backend contract: what MpqStorage and CascStorage implement and ClientFileSystem composes over