Review classification of SDEC activity

We currently classify SDEC with aedepttype = 05 but recognise that this might not work for all trusts.

From @paulseamer in https://github.com/The-Strategy-Unit/open-plan-docs/issues/14

There is uncertainty around defining and identifying SDEC activity. I think our default position has been activity type = 05 in ECDS. But we know not all trusts/sites record SDEC activity in ECDS. The latest guidance is especially confusing. Guidance dated 25 Nov 2025 states SDEC activity should in the future be reported in APC (not ECDS as previously advised); later guidance dated 10 Feb 2026 states that "the updated guidance will not apply to patient care delivered by Emergency Medicine within the Emergency Department, including ambulatory emergency medicine care, which will remain on Emergency Care Dataset (ECDS)". Can't pretend I can make any sense of this!
https://digital.nhs.uk/services/secondary-uses-service-sus/secondary-uses-service-sus-what-s-new#:~:text=A%20decision%20has%20been%20taken,this%20is%20implemented%20from%20Spring

Utlimately, I think however we choose to classify SDEC will be wrong for some trusts. The best we can do is to be consistent across all our NHP products with some supporting rationale but expect this not to work for all trusts, and be ready to change our approach as the consensus evolves.

Start by reviewing the current SDEC classification as aedepttype = 05 and the linked discussion in open-plan-docs issue 14. Read the cited NHS guidance on ECDS and APC reporting; done means agreeing and documenting a consistent classification rationale that accounts for differing trust practices and future guidance changes.