I was just about to reply to the other thread... but i'll hit this one first.
If you are creating a new PCMCIA or USB install device, you have to download the "install-media" file.
You use the "jinstall" when you are just downloading the file to the router to upgrade there.
The "install-media" is a binary image designed to become bootable, which is why you have to use the special process to write the image to the PCMCIA rather than just copying the file.
No. The install package version should only be used with the PCMCIA card (or USB). The install package will complete install all software onto the router and return the router to factory defaults. All configuration, logs etc will be lost. Once the installation is complete, the PCMCIA card should also be removed to prevent that the software is installed again from scratch when the router reboots.
I would recommend that you read through the release notes of the JUNOS version you are hoping to use as they contain information about upgrading and downgrading etc.
Here is the link to JUNOS 9.1 release notes.
JUNOS 9.1 Release Notes
I think people are getting confused by the very similar terminology here.
The install-media-* is what is required to build a PCMCIA/CF/USB that can be used to completely reformat a RE and return it to a factory default state. This image is a complete media image, therefore it has to be 'installed' onto the external card using the 'dd' command exactly as described on the webpage from which you download it.