Constructive Variation
A change to the works that has all the effect of a variation but was never issued as one — a revised drawing, an informal instruction, a change in required sequence. It is a variation in substance, and the entitlement exists whether or not the paperwork does.
The entitlement exists. Recovering it is a different matter, because the contractor now has to prove that the work changed, that the change was directed by the employer or its agent, and that the cost and time consequences followed — all without the one document that would have established it.
Which is why the discipline is to raise a confirmation of verbal instruction, promptly and in writing, every time. It converts a future evidential problem into a present administrative one, and administrative problems are much cheaper.