Aruch HaShulchan, The Laws of Tzedakah, 256:2

What is a public fund and what is a food collective? The Tur wrote, and these are his words: Every city that has in it a Jewish community is obligated to appoint tzedakah collectors who are known and trustworthy and who will call upon all the people each and every Friday and collect from each one of them what is appropriate to give, and the matter is fixed for them (the amount each person is to give is determined for them). They distribute the money each and every week and give to each and every poor person enough food for seven days, and this is what is called the public fund. Therefore, whoever has seven days worth of food may not take from it. And so, they appoint collectors who collect each and every day from each and every household bread and all kinds of food and fruit or money that is donated according to the immediate need. They distribute the collection in the early evening, giving to every poor person a day’s sustenance, and this is what is called the food collective. Therefore, whoever has a day’s worth of sustenance may not take from it - until here are his words. He wrote “they call upon all the people from Friday to Friday;” this does not mean that they collected only on Friday, but throughout the week in order to distribute on Friday. He wrote “and the matter is fixed for them” as if to say that concerning the public fund a donation of such-and-such was fixed for every head-of-household. However, for the food collective, there was no fixed amount. This discrepancy is because the public fund is for the poor of the city and you can count them, whereas the food collective is for all poor people (Baba Batra 8b); that is to say, poor people who are transient, so this cannot be fixed, because sometimes they are many and the other times they are few. [Translated by Rabbi Bruce Elder]