The factor is a simple multiplication factor. So a factor of 1.5 is 150%. The amount of distance that correlates to depends on the units of the spatial reference that the envelope is in. So if the envelope's SR is web mercator then it would ultimately be meters. If you use a factor of 1.5, it is not to say a simple value of 1.5 meters is the expansion, but it is saying the expansion is 1.5 times as many meters as it currently is.