The jump sequence scoring rule is on page 199 of the USFSA Rulebook, which can be searched online:
Jump sequences are evaluated as one unit by adding the base
values of the two highest value jumps, multiplying the result
by 0.8 and after that applying the GOE with the numerical
value of the most difficult jump. The factored base value of
the jump sequence will be rounded to two decimal places.
Well, this probably won't happen again, but my dd was newish at double lutz, and so at regionals, her coach decided to change it to single lutz axel sequence, it was more points than single lutz and safer than trying a double at that time. I assume it is sometimes easier to do a sequence.
Yes, if a skater is limited as to the number of loops and toeloops she is allowed to do, and/or the higher valued jumps she is able to do, the right sequence can give her more points than the other available alternatives, even though each jump in the sequence only gets 80% of its value.
BTW, there are no crossovers or turns allowed between jumps in a sequence (although a back double 3turn on the landing foot--with no stepdown onto the other foot--is allowed in a combination). So the hypothetical sequence of axel-lutz would be counted as two separate jumps if the skater landed the axel on the RBO edge, then crossed the left foot over the right in order to take off on the lutz. Also, doing a 3-turn before the salchow takeoff in a sequence would turn it into a separate jump element, which makes it very hard to put a salchow into a sequence now that a half loop-salchow is considered a combination.
Exact wording for these 2011 ISU rules:
JUMP COMBINATION
In a jump combination the landing foot of the first jump is the take off foot of
the second. The same applies to the third jump. If the jumps are connected
with a non-listed jump, the element is called a jump sequence; However halfloop
(or “Euler”) (landing backwards) when used in combinations/sequences is
considered as a listed jump with the Value of a single loop. When executed
separately, half-loop stays as unlisted jump.
No change of foot or turn is allowed at any time between the two
jumps, which must directly follow one another (except for touching down the
non-tracing foot in toe jumps).
In case of a touch down with the free foot without weight transfer and up to 2
three turns or no turns between the jumps in a combination, the element
remains a jump combination (however Judges will reduce the GOE because of
error). In case of more than 2 three turns the call will be the jumps performed
prior to three turns + combo in Short Program and + sequence in Free Skating.
If an Axel type jump immediately follows any other jump (without any hops,
mazurkas, unlisted jumps), this will also be considered as a jump sequence.
JUMP SEQUENCE:
A jump sequence consists of any number of jumps that may be linked by nonlisted
jumps and/or hops immediately following each other while maintaining
the jump rhythm (knee); there can be no turns/steps (not even as an entry into
a jump) during the sequence; there can be no crossovers or stroking. (Turns
are three turns, twizzles, brackets, loops, counters, rockers. Steps are toe
steps, chasses, mohawks, choctaws, curves with change of edge, cross-rolls,
running steps).