Write without Response - silabs-MarkOW/BLE_General GitHub Wiki
Core 6.1, Vol 3, Part G
Core 6.1 does not define "long characteristic" as such, but Write Long Characteristic procedure suggests it is longer than can be written using ATT_WRITE_REQ PDU:
Core 6.1, Vol 3, Part F
Core 6.1, Vol 6, Part B, 2.4
Data Physical Channel PDU
16/24 bit header length is determined by CP bit, indicating presence of CTEInfo octet.
Length is 8 bits, but payload is limited by MIC even if not present: