Oyster card daily cap include traveling by a multiple types of public transport in London, so if you mix them during a one day, the same price cap applies.
But there is also available a special daily cap for buses and trams. This daily cap applies when travel only by buses or trams in one day. So if you use only buses or trams to travel, there is a different (lower) price cap.
Oyster card daily cap is calculated over 24 hours for Pay as you go journeys, that start from 04:30 and end on 04:29 the next day.
If you use only buses or trams to travel in London, you may consider to get a Bus & Tram Pass.