Here's how this was developed: Many overlap brazing tests were made in labs around the world using different base metals, different filler metals and different amounts of joint overlap. These tests have shown that sheet-metal brazing using overlaps less than 2T failed in the brazed joint itself when subjected to tensile tests. As the overlap increased to more than about 2.3T or so, the failures invariably occurred in the base metal away from the joint and not in the brazed joint itself. So, the brazing community at that time decided to use (and publish) the 3T-overlap as a minimum overlap to ensure that failures would always occur in the base metal and not in the brazed joint.
Please note that the 3T-overlap already contains a safety factor of almost 30%. Therefore, a 3T-overlap, when the joint is brazed properly, should give full strength to any brazed joint and be completely leak-tight. Thus, a 3T-overlap is all that anyone ever needs in a lap joint! Not 4T, not 5T … just 3T is needed for a completely safe, sound joint!
Having said that, it was felt that if someone had a brazed joint that was approximately a 6T-overlap, and if it contained about 50% void content (of course, assumed to be randomly distributed throughout the joint), then it would still have a net-overlap (after deducting all the areas of voids) of about 3T. Since no one ever wants to have 50% void content or more in a joint, there should never be any need to extend the overlap from 3T all the way up to 6T, and so the brazing community suggested that people should not exceed 6T overlap in any of their brazing designs. Again, this was not because overlaps beyond 6T don't work, but because it is wasteful of brazing filler metal (BFM) to have to fill a much-larger-than-needed overlap. And it also wastes a lot of base metal to have to make an overlap that long! 3T to 4T is perfectly adequate for ALL braze joints, and overlaps longer than that are just wasteful!
Additionally, it has been found that when overlaps exceed about 4T, the percentage of voids in the joints rises rapidly since you are asking the BFM to flow through longer and longer joints. The void content does in fact rise rapidly as a percent of the total covered area. So, if a company has a void-content limit (such as a void-content limitation of 20%), it is even more important to limit the amount of overlap.