How do I assign specific terms and conditions to a segment?

It is not possible to assign GTCs to a segment. However, it is possible to assign them to a Customer Group. When you need to assign specific GTCs to certain customers, you must go to "My customers" and then "Create a new group".

Add the name of the group and save. A new "Customer Group Management" window will open. You can then assign the general sales conditions that correspond to this type of group.