Players can be subbed out for a 'like-for-like' replacement during a game if they're injured, too sick to continue or if a 'life changing' event happens in their personal life.
Injury & sickness replacements have to be signed off by the Chief Medical Officer. Think the life changing event replacements have to be signed off by both teams CEO's.
The Somerset subs during this round were Smeed for TKC (TKC suffered a bad hand injury taking a slip catch on morning of Day 1) and Vaughan for Goldsworthy (Goldsworthy did his hamstring on Day 2 I believe).
Smeed wasn't in the squad that travelled to Essex either - he had to travel over from Wales as he as playing in the 2s Vs Cardiff Uni the 3 days previous.
Interesting trial by the ECB - few concerns about teams potentially exploiting it. Personally I like it.