I'm not an expert on WWI maps, but I've been studying them a lot lately so here's my 2 cents.

The map is labeled "Artillery Instructions" and "Secret", which suggests to me that it's an operational plan intended for artillery crews. It's clearly marked with boundaries between battalions and appears to insinuate some sort of forward movement (~southwest to ~northeast). This suggests to me that it's an attack plan.

The incremental lines labeled with increasing numerals in intervals, often coupled with the word "lift", when I think of artillery, brings to mind instructions for some form of walking barrage - an artillery barrage that starts at a given location and then proceeds forward ahead of an attack to constantly provide cover to the attacking force by suppressing the defenders for an extended period of time along a relatively fixed corridor of advance.

The map is also marked with comments like "rest 15 minutes" or similar remarks, which are probably built in timings in the attack that the artillery would need to know about so as to avoid friendly fire or proceeding the bombardment too quickly.

The attack (which I'm assuming this is) covers a distance of approximately a mile over the course of about three hours, and is in the northwest of France near Belgium, just northeast of Cambrai. It seems that XVI Corps from the UK in WWI was stationed in Salonika, Greece, which would make this map being in their possession quite odd. However, a quick Wikipedia check (I know, I know) shows us that XVII Corps was formed in France in 1916 and engaged in offensive operations around Cambrai, which would make a lot more sense given the map is labeled "XVII Corps" in the center of the region of advance, despite being marked "XVI" on the cover page. I would suggest that an "I" got lost in metaphorical translation somewhere, which would explain the inconsistency.

The only thing I am having a hard time explaining is "practice trenches". That seems quite unusual given the extreme proximity to the front-line that this region would've had at the time, but also an attack over "practice trenches" - with artillery - seems rather odd. However, given that the rest of my explanation seems to fit quite tidily, I'd chalk that up to "weird terminology or something" and call it a day.

Hopefully that's a good place to start, and hopefully somebody more qualified than I am will be along shortly to fill in anything I screwed up or missed. Some of it is certainly still somewhat mysterious, so I have to guess that I haven't gotten it entirely correct.