With Tesco one supposedly gets no substitutions unless one ticks the box. There is a global box for allowing aubstitution for every item on the order if one wishes to use that and also individual check boxes for each type of item.
However, there is a Tesco policy to override and always substitute for some items, including milk and toilet rolls I was told. I don't know the full list.
However, one can usually override that override by explicitly writing not to substitute in the note.
The picker note of up to 55 characters including spaces, though a fewer-than-ASCII character choice allowed as, for exmple, a + sign is not allowed, can be used with or without using the substitution facility.
I am told that the picker note is displayed automatically to the personal shopper.
The picker note can be used to list alternatives and NOT alternatives, and also to request a choice where items are a mixture of manufactured items or are a food product that is variable.
For example, the notebooks are a mix of purple, white, black, so a picker note can be used to request a choice of colour.
For example, someone might request a mix of green bananas and yellow bananas, or, say, three large onions.
I wonder if the picker notes give the picker interest in the job, as the notes add variety to what would otherwise just be repetative.
I find that picker notes usually work.