10GBASE-R 64B/66B Encoding/Decoding Unusual Control Block Formats
Those are for ordered sets. Ordered sets are only sent in between packets, never within a packet. Control characters and ordered sets are not allowed within packets; if they appear inside a packet then it's an error and the receiving MAC should drop the frame.

0x2d block: four control characters followed by an ordered set

0x55 block: two ordered sets

0x4b block: ordered set followed by four control characters

0x66 block: ordered set followed by start and three data bytes. Basically the equivalent of 0x33, but with an ordered set before the start control character instead of four idles characters.

There are two ordered set types defined in the 802.3 spec: sequence ordered sets (O = 0x0) and signal ordered sets (O = 0xF). Signal ordered sets are used by fibre channel, but not Ethernet. The sequence ordered sets are defined in table 46-5, and include local fault, remote fault, and link interruption. The ordered sets would be sent in place of idle control characters between packets.