Portable TEMP Directory - If you create a directory called TempForPortableApps in the same directory as Start.exe ('next to' the PortableApps and Documents directories), the platform will use that as the TEMP directory for all apps it launches. Note that any local apps launched from the platform or its apps will also use this as TEMP, so use with caution!
what is the reason for caution? is it a drive/disk space issue?
Doesn't seem logical for it to be a security issue unless the warning is about "local apps wot aren't from us" which is surely presumed
perhaps it is just a warning to say "we're not exactly sure what might happen".