ANSWER: They are two very distinct items. Reserves are what you put away for savings at the end of the year. Carryover is money from the last fiscal period that will be used for operating expenses in the new fiscal period in the period between when the year starts and fundraising begins.
Whatever you carryover is determined in the budget you put together before the start of the fiscal year and have the membership ratify. You would allocate a portion of the money left over at the end of the year for carryover and part for reserves. If you have not touched your reserves for a few years, then you may not need to add to them that year, but instead use the money as carryover which would take some burden off the PTA for fundraising.