Petty cash is a small amount of money held for minor purchases that are impractical to put on a card.
Why it needs a system at all
The amounts are small, which is exactly why it goes unwatched, and unwatched cash is the easiest thing in a business to lose track of.
The loss is usually not theft. It is receipts not kept, so the spending cannot be substantiated and is not deducted.
The usual arrangement
A fixed float, say a set amount, held in one place by one person.
Every withdrawal recorded with what it was for, and a receipt kept.
Periodically the receipts are totalled and the float topped back up to the fixed amount by exactly that total. Receipts plus remaining cash should always equal the float, which makes checking it a single subtraction.
Recording it
The spending is recorded as the top-up happens, categorised by what the receipts were for rather than as one undifferentiated withdrawal — see adding an expense.
Recorded as a single line, the costs never reach the categories they belong to and every affected category is understated.
Whether it is needed at all
Less than it was. A card issued to the person who buys things captures the transaction automatically, with the description and amount, and needs only the receipt attached.
Where petty cash exists mainly because someone did not want to issue a card, a card is usually the better answer.
Cash taken from the business
Money the owner takes from a cash float for personal use is an owner's draw, not a business cost — see separating business and personal money.
The control
One person responsible, and the count checked by someone else — see fraud prevention.