Same here. Never heard of trenching. Just drilling holes to apply pesticide, or tenting. A neighbor had their house tented because the majority of their termites were actually inside the wood, not just chewing on the outside.
There are several types of termites most commonly found are 2 types found here in the US. Depending on where you live some areas only have one type the weather has everything to do with it, plus they really like damp areas. Here in the south we have Dry-woods and Subs, there have been areas that have Formosan as well.
There are many ways to treat. Do your homework, What type of contract did you sign.. one that includes repairs? or just re-treatment ? Should an inspection be done yearly? is there an annual renewal to keep said contract valid?
I will say this make sure that the company you have been with, has been around a while, and is licensed, not just a business license, but the techs that do the work are licensed and certified plus there is a certified operator at the company. Pest control companies have a ton of rules and regulations that must be followed, This industry has a lot of federal and state mandates, and are governed by, along with laws, that must be adhere to. the only other industry that has higher standards are Long term care facilities, Protect yourself. Your home is that largest investment and purchase most people make. You don't want to trust your home to a fly by night company, or just go with a cheaper price.
1) Drywoods which is the type that they use a tent to treat.
2) Subterranean termites live under ground and forage for wood, they are the ones that build the kinda tunnels that you can see something going up the side of the wall, they like a damp area.
There is a 3rd) - termites which is a Formosan( they have been found here in the US) they are like a super termites they can consume wood very rapidly, and can do alot of damage in a very short amount of time.
While they are trenching and treating, you will want some type of bait station place in the yard, so that they whatever company you are using, can normally check quarterly to make sure what is going on.
I worked for a pest control company.