The sequence of octets within an SPDU and the sequence of bits within an octet
are defined for each SPDU in 8.3, with the additional convention that where the
text refers to bits within a two octet field and the bits are numbered 16 to 1,
then 1 is the low order bit and the octet containing bits 16 to 9 precedes the
octet containing bits 8 to 1 in the SPDU.
第一个逗号之前的，能明白。大概是说，“SPDU中的各字节各位所代表的意义在8.3中有说明。”之后的就不太理解了。主要是有一个where和then。不知道应该怎么理解。