apps & files in .zip are not 'portables' (.paf) - they are just 'standalone'
Maybe there are other specifications, but I prefere: PortableApps.com Format™ 3.0 (2012-05-11)
PortableApps.com Format is a simple specification that governs the file and directory layout as well as operating behavior of portable apps. File are distributed as easy-to-use .paf.exe™ installer files. The specification is broken into sections for easy reading.