Very much worth carefully reading May’s leaked letter to the DUP in this top @SamCoatesTimes scoop thetimes.co.uk/article/9023b6…
If you cut through the semantics and May’s typical obfuscation, and read it alongside how the EU described the backstop and “backstop to the backstop” to EU27 last week (buzzfeed.com/albertonardell…), this is what I think it means, a short thread:
May is, in short, prepared to accept the EU backstop - a bare bones UK-wide customs union and a Northern Ireland only backstop to the backstop (a customs union doesn’t remove the need for all checks) - and she is trying to sell this idea to the DUP
May says in the letter that a backstop would only be temporary as it’s defined by article 50 / withdrawal agreement. Technically this can be true: it wouldn’t last forever, but only until and unless there is another solution in place
Worth also reading N10 comment to @SamCoatesTimes news story - Downing Street specifically talks about customs territories. 
As ever, one challenge of interpretation is that May isn’t always clear (see this excellent @tconnellyRTE thread ) - and this, when and if there is a deal, will be a problem
Finally, worth keeping in mind that, even if agreed by negotiators, a backstop that includes a bare bones customs union still needs to be approved by EU27 (and, then of course, the whole thing by UK cabinet and parliament)
One extra point, also worth keeping in mind: the backstop isn’t a bridge to a future deal, it’s an insurance policy (as May correctly describes it in the letter) to avoid a hard border under all circumstances and scenarios, now and in future
