All the majors follow tariff rules. One such, is rule 120.20 this is an endorsement to another carrier that follows the following criteria:
1. Same day departure, any flight cancelled due to weather, mechanical, and ATC thinning.
2. The city pairs must match for example; you can't use this rule if the cancelled flight was taking you to SFO, and the new carrier is flying to SMF.
3. There must be residual in the ticket. It must have some value. Frequent Flyer mileage tickets, or free tickets cannot be endorsed.
Rule 120.20 simply grants what ever the ticket cost was to the new carrier. If OW of your ticket was $90.00 then the new carrier is paid that.
Now if a carrier reroutes to a different city, a different day of departure, or if the ticket is free then the carriers use RULE 240. Now this is based on percentages of full fares. Some airlines have different agreements with different carriers. If DL excepts a UA ticket with rule 240 then UA pays DL 30 percent of a full OW fare. Some carriers like AA are very stingy and require 50 percent of full fares from other carriers. Some have argreements at 20 percent also. So when airlines like NW went on strike last year, DL profited greatly. In cases like strikes most carriers make arrangements for ticket acceptance without endorsements.
Well this is probably a little more than you wanted, but at least informative.