I've seen this restriction - Q8 - discussed somewhere else before.
What it appears to be doing is defining earliest start times for stations on the Cardiff Portsmouth axis and its various timetabled branches. Note that the list doesn't include any stations from Exeter via Yeovil Junction towards Salisbury, it definitely seems to be aimed only at the
GWR▸ operated routes and services.
http://www.nationalrail.co.uk/Q8I think the note at the bottom of the page is pertinent - it gives an explicit permission to depart earlier from other stations:
"
Off-Peak tickets from stations not listed are valid for connections into trains departing as shown above. "
You'll see that the earliest outward departure from Salisbury is 0850, and that is the exact train that the 0641 from Exeter St Davids connects into.
If you are still worried print off the restrictions page.
Paul