It's the document that the government produces telling the railways what it wants them to do in the next control period. It's linked to the Statement of Funds Avalible (SoFA) which is the document that tells the railway how much money there is available which then informs what makes it into HLOS. It's a government led document but normally consults with various industry bodies, local organisations and councils to determine what is needed/should be done.
At least that's my understanding of the term.