I don't think any sources are that specific (someone will correct me, I guess?)...

What we can guess from basic mechanics and modern combat reenacting is that shields will be suitable for different 'bashing' obviously.

With small buckler on the end of an arm, one can actually punch someone, from different angles, into different places, with different effect.

With Roman style large shield, actually putting your whole weight behind the shield and slamming into something would be more 'natural' - even though strikes with edges and boss in different ways are also possible.