A more thorough explanation:

A memorandum of alteration is a legal document used in English law by a patentee to disclaim certain rights to avoid losing the entire patent. This is typically done when part of the invention is not new or useful.

For example, if a patent is granted for two inventions but one of them is not new or useful, the entire patent would be considered defective. In this case, the patentee can use a memorandum of alteration to disclaim the non-new or non-useful part of the invention and maintain the validity of the patent for the other part.

Another example could be a patentee who realizes that their invention infringes on someone else's patent. They can use a memorandum of alteration to disclaim the infringing part of their invention and avoid legal action.