What do you mean by "customs clearance" and "single customs clearance" in the tourism industry?

L signing in from the mainland to Hong Kong must be "cleared". "seeing off" is just a common name, but it is actually called "going in and out with the team", also referred to as "listing". You don't have to have a travel agency to send you, but you must have a group ticket to Hong Kong and Macao, or you won't pass the L-sign. With this list, you can go through the customs. This list was only checked at the border inspection in the Mainland when going to Hong Kong and Macao from the Mainland. Hong Kong and Macao border inspection does not check whether the L-sign brings a delegation. So if you go to Macau from Hong Kong, you don't have to make a list, and you don't have to come back. Just come back by yourself.

Single customs clearance means that there are only customs clearance services and no other tourism projects.