SPXpm is a fully-electronic version of the CBOE's flagship S&P 500 Index options (SPX), the most actively traded U.S. index option product. The options launched Oct. 4, 2011 on C2, CBOE Holdings' electronic options exchange, which began operating in September 2010. The product is identical in structure to the traditional SPX index option except that it has a "p.m" settlement.  It received SEC approval (on a pilot basis) on Sept. 2, 2011. 
The contract has a 100 multiplier and is ten times larger than SPY (SPDR ETF) options. It is cash-settled and has European-style exercise, like SPX and most other index options.
On Nov. 29, 2012, CBOE Holdings announced it would transition its SPXpm product from the C2 Options Exchange to Chicago Board Options Exchange (CBOE), where it will be traded on CBOE’s Hybrid Trading System. The transition will consolidate the company’s entire S&P 500 options product line on one exchange. The pm-settled SPX options (including SPXpm, SPX Weeklys and SPX Quarterlys) will trade under ticker "SPXPM" and the flagship SPX option, which is a.m.-settled, will continue to trade in open outcry under ticker "SPX."
Market makers in the contracts are charged 17 cents per contract while brokers and active traders deemed professionals pay 40 cents. These groups will also pay a 10-cent surcharge per contract to offset C2’s licensing fees for the index. 
- CBOE Holdings' SPXpm to Launch on C2 on October 4. CBOE Holdings.
- SEC Approves Launch of All-Electronic S&P 500 Options. The Wall Street Journal.
- C2-SPXpm Micro Site. C2.
- CBOE Holdings To Transition SPXpm From C2 To Hybrid Trading On CBOE. CBOE Holdings.
- CBOE Sets Fees for Electronic S&P 500 Options on C2 Exchange. Bloomberg.