A lightweight and comfortable harness for recall training and car attachment, also used for walking dogs.
However, I do not believe they should be used for walking to heel.
I have written an article explaining that using a harness or a halti causes pain and discomfort to your dog.
Type into Google The Importance of the Right Lead and Collar I will probably be the top result
Choosing a lead collar or harness is vitally important it explains that you should always use a lead and collar to train any dog.
If a trainer advises you to use a halti or harness to train your dog then walk away.
That person does not know how they work and why they cause discomfort to the dog
On top of that, they are clearly incapable of training a dog to walk to heel.