Petty Cash Book is that the company have cash on hand and used for paying small (petty means little amount) amount expenses. It should have a policy to determine which item should be paid by petty cash.
Those payments under $150 can be reimbursed by the petty cashier.
$150 is the policy to determine that amount smaller than $150 are paid by petty cash. Amount >=150 cannot be paid by petty cash.