Basic Logic of the Index
S&P500 VIX Mid-Term Futures Index is an investable index that tracks the outcome of holding a long position in mid-term VIX futures. Long positions are held in the fourth, fifth, sixth, and seventh VIX futures contract months. As the time to expiration of the futures shortens, the position in the fourth month is being constantly rolled over to the seventh month, maintaining constant maturity of 5 months.
Mid-Term vs. Short-Term VIX Futures Index
The logic of the S&P 500 VIX Mid-Term Futures Index is the same as that of the better known S&P 500 VIX Short-Term Futures Index, with the only difference being the target constant maturity (1 month for the short-term index and 5 months for the mid-term index) and the futures contract months used. Both the short-term index and the mid-term index are part of the S&P500 VIX Futures Index Series, which also includes similar indices with 2, 3, 4, and 6-month maturity.
S&P500 VIX Mid-Term Futures Index is widely used as a benchmark for various long volatility strategies and also as the underlying index for volatility ETFs and ETNs which track the middle (or the middle to longer end) of the VIX futures curve, such as VXZ, VIXM, or the inverse (bearish) ZIV. Nevertheless, these mid-term ETFs/ETNs still have much lower assets and trading volumes than the ones targeting the short end of the VIX futures curve (using the S&P 500 VIX Short-Term Futures Index).
All these exchange traded products are a popular way of getting exposure to the VIX (CBOE Volatility Index), which is not investable itself.
S&P 500 VIX Mid-Term Futures Index Official Information
Official page of the index and factsheet: