What happened

Two Wednesday winners on the exchange were email security specialist Mimecast (MIME) and bank holding company Trustmark (TRMK -1.25%). The former rose by nearly 8% on the day, with the latter increasing by almost 9%. Both gained on the back of upcoming changes to a set of crucial stock indexes.

So what

After market close on Tuesday S&P Global (SPGI 0.12%) -- the company that owns and manages the S&P suite of indexes -- announced one of its periodic realignments.

A woman looking at a set of indexes and graphs.

Image source: Getty Images.

Among other adjustments, one component company on the S&P MidCap 400 index (SNPINDEX:^MID), movie theater chain operator Cinemark Holdings, is being shifted to the S&P 600 SmallCap index. This is because, in S&P Global's words ,it "has a market capitalization that is more representative of the small-cap market space." It is to be replaced by Mimecast in the S&P MidCap 400 index.

Using the same rationale for Trustmark, S&P Global is moving it from the S&P MidCap 400 to the S&P SmallCap 600, where it will displace Sykes Enterprises, another acquisition-to-be (in this case, by privately held Sitel Group).

All changes are to be effective prior to market open next Monday, Aug. 30, S&P Global said.

Now what

The difference between the Mimecast S&P 400 MidCap index inclusion and Trustmark's move to the S&P SmallCap 600 index is that the former isn't shifting from another S&P lineup.

But the result is similar -- the fresh inclusion of Mimecast on a high-profile S&P roster should garner it more attention from both index funds and investors. Meanwhile, Trustmark stands to benefit from its continued presence in the index family's limelight.