A targetGeom channel is designed to work as morph holder, that is it can contain deltas (the elements that comprise a morph target). A valueParm channel is usually used as a master channel in an ERC (Enhanced Remote Controle) relationship. A valueParm channel can't contain deltas, therefore it can't work as a morph channel. Most channel types can be used as master (or slave) channels in an ERC relationship, however the trackingScale for most channels is hard coded, the trackingScale for a valueParm channel can be set to any value desired, this makes it the best choice for a master channel in some circumstances.