I found some mention in Openreach's "EIP" documents, and it appears to relate to the 10th percentile, but also seems to be being added as part of the G.Fast rollout.

The EIPs usually document changes that will impact the way that CPs interact with Openreach.

"R3250" was apparently released last September, "R3300" was last November, and "R3350" is due next week.

R3250 says

In reference to www.openreach.co.uk/orpg/home/updates/eipcommunications/downloads/E...:

G.Fast LD: VDSL to G.fast technology change modify via handback

R3300 says

In reference to www.openreach.co.uk/orpg/home/updates/eipcommunications/downloads/E...:

Downstream handback threshold (10th percentile) on eMLC (to display the handback threshold on eMLC and use it to judge whether a line is eligible for handback in T2R)

R3350 says

In reference to www.openreach.co.uk/orpg/home/updates/eipcommunications/downloads/E...:

G.fast: Full Test on Handback support

R3350 also includes an item as part of SOGEA:

In reference to www.openreach.co.uk/orpg/home/updates/eipcommunications/downloads/E...:

Automated Handback Eligibility Check

Hence why asking, so we don't have to play whack a mole with guesses

Only AndyHCZ will know more of this. Ask him.

I only just noticed this and wondered the same. There's an ISPreview article here:

http://www.ispreview.co.uk/index.php/2017/01/bt-whol...