The last time I thought about TCP header bits was in 1998, so all the revision history and technical content is opaque to me.  I have a few nits on formatting and language, which may be entirely off-base because of my lack of context.

Nits:

1. Introduction - why is the single sentence beginning "The bits in offsets 0 through 3" indented that way? Should it have "Note:" or something in front?  Is the middle of the section the right place for it?

2. Section 3 - "for each TCP control bit, there is a bit in this set" - took me a bit of mental work to parse. Upon re-reading, I think you could maybe just lose this: "This information is encoded as a bit field; the bit is set to 1 if any observed packet of this Flow has the corresponding TCP control bit set to 1."

3. Section 3 - This section makes regular use of the verb "export", does this in effect mean "set"? Should that be made explicit?

4. Section 5 - " an exporter has to report " - should "has to" be "MUST"?