From the CBOE website:
PROPOSED SHORT-TERM S&P 500 INDEX OPTION PRODUCT SPECIFICATIONS
Symbols:
JXA, JXB, JXD, JXE (Weeks 1 through 4 root symbols)
Underlying:
The Standard & Poor's 500 Index is a capitalization-weighted index of 500 stocks from a broad range of industries. The component stocks are weighted according to the total market value of their outstanding "free float" shares.
Index Components
Contracts Listed:
New series of short-dated Standard & Poor's 500 index options are listed each Friday morning and expire one week following their listing date. The third character in the root symbol denotes the expiration week of a given month. For example, JXA is the symbol for S&P 500 short-term options that expire on the first Friday of the month; JXB is the symbol for S&P 500 short-term options that expire on the second Friday of the month, etc. No more than two classes of short-term options based on the same underlying may be listed at the same time. For example, JXB is listed on the first Friday of a month, the day on which JXA options expire.