Transport block size — How many bits fit in the container
Bits into a codeword
Where it sits
What it is
The transport block size is how many bits fit in the container — computed identically at both ends from the same inputs, with nothing in the DCI carrying it.
Four steps (TS 38.214 clause 5.1.3.2): count the resource elements per block after subtracting DM-RS and configured overhead, capped at 156; multiply by rate, modulation order and layers for an unquantised $N_{\text{info}}$; then quantise — via a 93-entry table below 3824 bits, and by an arithmetic rule above it that makes the result divide evenly into code blocks.
Why quantise at all? Because a retransmission may use a different allocation, and combining only works if both transmissions agree on the length. A coarse ladder means a modest change lands on the same rung, and the two ends keep agreeing without any signalling.
Read on
The full treatment is in nr-pdsch — this page is the definition and the map reference; that note is where the subject is actually developed.
Before this concept, the hierarchy says to learn the following — the full chain, in order: