indication welding design

[Cyborg77]

Good morning to all,
Have any of you ever seen this welding indication? Can you help me?
I found no reference to the highlighted part.

Thank you.

 

[TETRASTORE]

Hypothet:
20 = length of the welding tract
+3/0 = tolerance
lg = lenght groove
In practice it is a comment that the tolerance +3/0 refers to the length (lenght) 20 of the welding cord (groove).