The IAD-FRA segment of UA 916 is always a 744 ... aircraft substitutions on international routes are extremely rare.

The SEA-IAD 767 could head off to any number of 767 destinations in the evening: EZE, GRU, ZRH, FRA (as UA 952), LHR. It could even head to SFO as UA 915 or ORD as UA 951.

If your SEA-IAD segment is operated by a 752 rather than a 763, that means that the IAD-SEA the night before had an aircraft substitution, most likely to avoid a delay on an international flight.