In January 2016, the EBU published its updated loudness recommendations.
The main EBU R 128 document has not changed.
Version 1.0 specified that among other parameters either the
maximum short-term loudness should be below -18 LUFS or the
maximum momentary loudness should be below -15 LUFS.
In the current version 2.0 the maximum momentary loudness option
got removed.
The reason for this is given in the (currently only available in print, not online) EBU tech-i magazine issue 27: "...for the sake of simplycity. ...tests showed that using MaxS of +5 LU [= -18 LUFS] is highly compatible with MaxM at +8 LU [= -15 LUFS], the former leaving a bit more dynamic freedom."
Additional test signals are provided to verify that a meter complies to the specifications. The LUFS Meter passes all these tests. The exact measurements are provided in the appendix of the LUFS Meter manual. The appendix in the LUFS Meter manual provides the results for these tests.
Also, the displayed value of the loudness range shall be indicated as not yet stable within the first 60 seconds of a programme. The LUFS Meter indicates this with a blinking LRA value during these 60 seconds.
The minimum overlap between consecutive measurement blocks has been raised from 2 seconds to 2.9 seconds.
The LUFS Meter already uses 2.9 seconds since version 1 (released in May 2015) because measurements showed that other manufacturers of loudness meters also have used mostly 2.9 seconds at that time. The official recommendations do now reflect this common practise.
This document got changed dramatically. It is a quite good introduction to loudness measurement and how to conduct it.