plug or slot weld symbol
Slot & Plug Weld Symbols - Weld Guru
The spot weld symbol is simply a circle that may be placed above, below, or centered on the reference line. When the symbol is centered on the reference line this indicates that there is no side significance.
The Different Weld Symbols Explained: Welding Basics
My slot weld call out was based on a simplistic definition that while fillet welds maybe be placed around the inside of a slot, if you completely fill the slot than a slot weld symbol would better describe the weld.
Nghĩa của từ Slot weld - Từ điển Anh - Việt
The slot weld symbol is the same that is used for plug welds. The symbol will not show a diameter symbol before the size however. The size of the weld will be the slot width instead.