The CT measures the current that carried by the wire that passes through the center of the CT. They come in different sizes to handle almost any current up to 1600 amps. The most popular ones are the solid core ones that handle up to 200 amps each. Solid core CTs require that the wire carrying the load be disconnected, put through the center of the CT, then reconnected. Split core CTs open so you can go around live wire and re-attach.
Generally, one CT is used on each hot leg. In some cases, two or more CTs may be used on each leg if the wires are places such that they will not all go though a single CT. If using multiple CT’s they must be run in parallel and attached at the meter terminal accordingly. Since the CT leads can be extended 500 feet or more, this sometime will save the expense of a second sub-meter. Extreme care must be used when doing this because if not done properly, the CTs will provide the difference between the current in the two wires instead of the sum of the currents. Detailed instructions are in the manual.