Sometimes it can be just a transfer of shares from one account holder to another without disrupting the sp.

Internal Cross trading happens all the time, and then there is the Upstairs trading, that happens in the backrooms of brokerage firms. Also, there are alternative brokers specializing in trade matching, also called dark pool trading.

Historically, in an upstairs trade buyers and sellers would negotiate the price and conditions of the trade in the "upstairs" rooms of a brokerage firm. Some exchanges prohibit upstairs trading.

The dark pool gets its name because details of these trades are concealed from the public, clouding the transactions like murky water. Some traders that use a strategy based on liquidity feel that dark pool liquidity should be publicized.