The phrase "most airline partners" means that an airline has established relationships and agreements with a large number of other airlines to provide services, such as shared flights or frequent flyer programs. Full definition
Since most airline partners transfer at a 1:1 ratio, that's like earning 1.25 miles per dollar spent on the card for all spend. (viewfromthewing.boardingarea.com)
So essentially can transfer to most airline partners of SPG for a 1 to.625 rate at 20,000 SPG points. (flyertalk.com)
For example, American Express Membership Rewards transfer at a 1:1 ratio for most airline partners. (upgradedpoints.com)