Alloy sizes

Guys,

I was looking through the What wheel specifications (eg PCD, ET) are fitted to Skodas? FAQ

I assume the ET is some sort of offset which is related to the PCD in some way ? Or is it ?(How come the ET for the 15" & 17" Octavia Alloys are the same while the 16" are different).

Also what does the J & H refer to for the Octavia ?

Lastly, Is it okay to use a 16" wheel as a spare if your running 17" wheels ? I wouldn't think there would really be a issue for short periods as some makes use space saver wheels but then I wondered why Skoda put a full size 17" spare wheel into cars fitted with 17's

See here