The tail communicates so many things and when flexible enough it provides one more way to to signal bits of information.
In fact, I have witnessed my brindle casually draping a tail across the back of it’s roomie as they stand together surveying a view side by side.
I unfortunately miss photo ops of these fleeting moments though because of the randomness of the behavior.
This photo however, is a per chance lucky snapshot of a similar behavior as one canine wraps its tail around neck of the other. My brindle could have dropped its tail to a straight position or moved away, but instead chooses to seemingly curl it’s tail up. (BTW no dogs are in heat and sp/neuter is the norm here.)
It is interesting behavior none the less, and I won’t make more of it than to say, if they did not get along so well I suspect that they would not have such close contact and mutual camaraderie. It’s great when dogs can have a “BFF” too!