From other posts tagged Six-Set —
The six-set also underlies the 21-point projective plane PG(2,4) —
PG(2,4) may be used to construct S(5,8,24), also known as
the large Witt design. Some related research . . .
On four codes with automorphism group PΣL(3,4)
and pseudo-embeddings of the large Witt designs,
by Bart De Bruyn (UGent) and Mou Gao (UGent),
(2020) DESIGNS CODES AND CRYPTOGRAPHY. 88(2). p.429-452.
Some background reading —