It is written in the Conditions of Carriage of most airlines (probably all of them). The difference is that a dated segment becomes an involuntary reroute, while open segment is not.

I'd still like to hear from someone what possible benefit an open segment has in the current e-ticket environment, where changing dates is now so easy. I did use open segments on paper tickets, with good reason, but now I cannot see any value to it at all. Actually most airlines have also made changing dated paper tickets much easier than it used to be (BA will even take it unendorsed for non-BA vouchers, guess they are hungrier now for revenue).