This document is commonly seen in soldiers' files in series WO 364 (Soldiers' pensions from the First World War). It's a Ministry of Pensions' form rather than an Army Form, as such, hence the MP title. In the absence of other documents though - or read alongside a partial service history - it can provide useful information such as the year of birth, date of enlistment and degree of disablement.
In this particular case, the date of enlistment is noted as a date of re-enlistment because Arthur Nixon (no relation) had originally enlisted for four years with the 7th Battalion Northumberland Fusiliers. That was in 1912 and he'd served in the UK until March 1916. In this man's case, only this Ministry of Pensions card and his original four-page attestation survive and so this card is useful in that indicates subsequent service with the Labour Corps and records that his degree of disablement - aggravated by war service - was still at 20 per cent in 1921.
The images on this post are Crown Copyright, The National Archives.