Welding Procedures Specifications(WPS) for Beveled Tee-Joint

What does the applicable fabrication specification say, either directly, or by reference to another welding procedure qualification standard?

Assuming you're working under ISO standards:
A beveled T-joint falls under butt welds (doesn't matter if it's full pen or partial pen, it's a butt weld).
For butt welds and in most cases, tensile, bend and impact testing is required. This means you'll have to perform a butt weld on plate, so you can provide the necessary test specimen. Usually for these kinds of joints, macro's are also recommended. This means you'll also have to do a pre-production weld test.

Make sure if you qualify fillet welds in a T-joint (one side a single layer, one side a multi layer), you take into account the double of the heat input if you're welding the first layer simultaneously. I try as much as possible to separate both tests (sl/ml), unless they are to be welded simultaneously in production as well.

Thank you. The applicable standard is ISO 15614-1.Assume that the vertical base material is plate A and the horizontal base material is plate B.
(1) For the base material A of the T-joint, it is a butt weld. For the base material B of the T-joint, what type of weld is it?
(2) Could I use butt weld in one plate to cover the T-joint between Plate A and Plate B? If possible, what is the thickness range? (The thickness of the butt weld in one plate, covering the thickness vertical of plate A and horizontal plate B of the T-joint?)
(3) If the butt weld has already been tested to qualify T-joint, is it necessary to make a separate T-joint to be tested?
Thank you very much

(1) butt weld
(2) depends on the base metal. if A and B are in the same group, then yes. When qualifying a butt weld in plate, the thickness range is based on the thickness on the plate and goes for both parts of the production (T-) joints.
(c) depends on the requirements. If I were either the fabricator or the client, I would insist on at least a supplementary macro even if it's not required.