Cursor (round)
A counter that moves to a new HMAC output when a game needs more random bytes than one 32-byte hash provides.
One HMAC-SHA256 output is 32 bytes, which gives 8 floats at 4 bytes each. Games like dice and limbo need only one float. Others, such as plinko with 16 rows, mines with many mines, or card games, need more.
How the cursor works
In the common scheme, the message includes a round number: clientSeed:nonce:round. The first 32 bytes come from round 0. When those run out, the generator computes a new HMAC with round 1, then round 2, and so on. The cursor tracks the position in this stream of bytes. Some documentation calls it the cursor, others the round or current round.
Why it matters
If you verify a 16-row plinko drop using only one HMAC, you will get the first 8 rows right and the rest wrong. A good verifier handles the cursor automatically.
Worked example
A mines bet needs 24 floats. That is 96 bytes, so the verifier computes rounds 0, 1 and 2 (32 bytes each) and reads the floats in order across them.