Many shops have developed "Standard Shop Practices" where the minimum size of fillet welds are established based on the thickness of the thinner of the members being joined. In the case of a sheet metal shop where light gauge metals are welded, the size of the fillet weld is typically equal to the thickness of the material being joined.
Other companies modify the basic AWS welding symbols to suit the particular conditions of their industry or company. Aerospace companies I've worked with as well as a few pressure vessel fabricators have developed their own "standard symbols" for use by their in-house workers. In some cases they are actually "published" so that everyone can interpret them the same way. This is fine for in-house work, but it creates nothing short of havoc when they try to subcontractor work to outside vendors.
When I went to work for one company I told them their welding symbols didn't follow AWS conventions. The response was that it was alright because everyone in the company knew what they meant. I said it would be a problem if we were to subcontract any work, but the response was, "We don't subcontract any of our work." Guess what, within a year they were trying to subcontract some of their overload and several of the vendors returned the bid packages unopened. When I asked why, their response was, "We've seen your drawings before and we won't even bid based on the information in your drawings, they are incomplete, the welding symbols are wrong, and the dimensioning system you use doesn't provide use with the information we need or they are missing altogether. Hum, just what I had said.
I attached a sketch of an example of a nonstandard welding symbol that I've seen used by several designers. Most people can interpret it, but it is not the normal way of specifying lengths of welds.
Best regards - Al