He says in registration_utils.cpp line 263
```
  // DefaultHandling has a value of 0 or 1, so we can store it directly in the pointer. Not perfect, but
  // harmless if done right.
```
I think it's because he doesn't want to attach an entire object to the transaction (I suggested he should, but he argued against it: "I'll use raw PJSIP transactions, because the UACTransaction and UASTransaction classes are too tightly bound to proxying to be easily generalised out."). I don't think this is very nice, though - suggest you email him / raise an issue so he can address when he returns.