The external wall finish of a timber frame kit serves no structural purpose and is required simply to provide weather protection. So, just so long as the material specified is weather-proof it can be used.
Whilst conventionally the external wall finish will be either rendered blockwork or brick / stone other finishes such as larch boarding (horizontal shiplap, vertical channelled or vertical board on board) and plastic coated profile sheeting could be specified.
Larch board and plastic coated profile sheet finishes are ideal for DIY self-builders since they require no particular skills to fit. Also, since they are fixed directly to the timber frame, foundation design will be simplified.