SAF has to be a 'drop-in' fuel

A jet engine certified decades ago must burn today's SAF without modification. So every pathway has to produce a fuel that meets the same spec as fossil jet — certified under ASTM D7566, then blended and re-certified as ordinary Jet A-1.

That single constraint explains the whole pathway zoo: there are many ways to turn carbon into jet-range hydrocarbons, and each gets its own ASTM annex and blend limit.