Basicly the same as the PKG file format before but with one change. The pkg is not a own container which is additional encrypted anymore. It is a renamed non encrpyted .zip file which holds a copy of the ux0:bgdl/t folder which can be obtained at download-time
How to create your own pkgs
Download something from PSN. then open VitaShell After wait for you to get the "Download completed" notification Then goto ux0:bgdl/t and there will be a folder named something like 000001 add this file to a .zip and rename to .pkgs.
Now you can use your pkgs and install it like every other one too.
Attemption ! This will not remove any DRM !