The activities you can register your hours on can be freely defined by you. After all: not all projects have the same activities. You are probably not interested in a "travel" activity when you are purely working from home, but you might for instance want to include a "bookkeeping" activity if you are self-employed. You can also split up your registered working hours over two separate sub activities and place them under the same main activity; sub activities "work from home" and "work at client" for instance can be placed under the main activity "work". This can be very useful when some activities have different billing characteristics and also need to be reported in total.