Martin,
TVH actually has 3 types of linkage:
Brand - Which links episodes (generally Brand is analogous to Show, i.e. The Simpsons, The X-Files, etc..), almost NOTHING supports this but its there for a) future proofing and b) because I do have a UK source that does provide this info.
Season - Which links episodes. This is pretty straightforward and is what most people think or as a Season/Series, i.e. The Simsons Season 1 (2,3 etc...). These two levels will be familiar to anyone that buys DVD box sets of TV shows and/or uses XBMCs TV shows library.
Series Link - Which links broadcasts. This is a broadcast/DVB centric model, most of the time a Series Link is analagous to a Season. However the model used by the broadcasters a) does not require this (its occasionally possible to have random thinks linked, maybe for some seasonal specials week or something) and b) often links a specific set of broadcasts (i.e. there might be one series link specifically for the prime time airing and a completely separate link for the repeats).
For the most part, because its not explicitly listed as a Season, things are mapped into Series Link's.
With regards to the specific processing of dd_progid, the values break down into 3 parts, TTX*.Y*.
TT = Show type, it can either be SH/EP or MV I think
X* = Season reference
Y* = Episode reference
For MV/EP the full reference string is used as the ID of the episode. SH does not provide an episode ref, so you won't get dup detect on those. Typically though these are things that don't repeat like news programmes I think.
For EP the ref up to the . is used as season reference.
And having just typed that I think there is a bug in the code ;) I think that last statement should be true for EP and SH. so I need to look into that.
Maybe if you have an idea of how you will get your references and what they will look like we can consider whether this is approach is in fact the best one to use. Personally I don't like dd_progid, however its better than nothing so it would be bad form of me not to process it.
Adam