The actual entry in the Sequence Number field is based on the number of bytes in the TCP data field, i.e., because TCP was developed as a byte-oriented protocol, each byte in each packet is assigned a sequence number. Because it would be most inefficient for TCP to transmit 1 byte at a time, groups of bytes, typically 512 or 536, are placed in a segment and 1 sequence number is assigned to the segment and placed in the sequence field. That number is based on the number of bytes in the current segment as well as previous segments, because the sequence field value increments its count until all 16-bit positions are used and then continues via a rollover through zero. For example, assume the first TCP segment contains 512 bytes and a second segment will have the sequence number 1024.
I looked it up in a dictionary and I don't think there was a meaning that even remotely matched what I've got there in that paragraph.