Most jurisdictions have requirements that must be met in order to have a formally valid will. This is an attempt to prevent fraud, undue influence, substitution or mistake from occurring due to the fact that the person who created the document is not available to be questioned when an executor is applying for probate. I had an earlier post discussing formalities https://heirsandsuccesses.com/2015/08/09/can-i-get-a-witness/
In some jurisdictions Courts can decide to admit into probate a document that does not meet the formalities required for a Will if it is satisfied that the deceased intended the document (or part of the document) to constitute, amend or revoke (whether fully or partially) their Will.
Courts have taken a broad definition of ‘document’ Including a tape recording, video recording, and recently a message created on the electronic notepad of an iphone by the deceased person being admitted as a will. However these are exceptional and usually,the court admits a written document that fails to meet the formal requirements of a Will (for example it is incorrectly witnessed).
To assist in making this decision the Court may take into account that at the time the document was created the deceased intended that it would constitute their Will including any actions or statements that were made by the deceased around the time that this document was created.