RE: Clarification>>>>It is not uncommon for large houses to find themselves with a client who wants to get rid of a large block of shares whom they can match with another client of theirs who would be interested in buying that same block. There is an obvious benefit for the house of collecting the commission on both sides of the trade as opposed to having to share it with another trading house.
It can also happen that a financial institution finds itself with a holding in one of its funds (eg. mutual fund) which is no longer a good fit or for which they simply have too many shares and want to reduce the exposure. They might decide to shift it over to another one of their funds where it's a better fit, or even right outside the fund to one of their clients who is interested in it. Thus a cross takes place.
There are many reasons these trades take place but they are essentially meaningless, and they are handled carefully so that they rarely have any effect on the stock price. I never pay any attention to them since they are typically an indication of absolutely nothing.