@papershuffler
What is the procedure for issuing them? Are they issued by request of the patient, or is it down to the doctor to suggest?
That depends. Usually a physicians decide wether one is
able to work or not. He judges according the diagnosis he made
and what a patients tells him during his visit.
I.e., if one has a severe flu with fever he will write a sick note without
asking him for that.
But if one shows up just with a cold a patient may ask for this because
one explains him, he/she would need some days off from working because
of having problems at work.