capital calls
Investment firms raise money from Limited Partners. Investment firms primary goal is optimize its return on investment to its partners including Limited Partners. Because of the time value of money, the concept of "return on investment" involves both the total amount of money returned and the timing of that return. A $1 return tomorrow on an investment of $1 may be preferable to a $100 return in 100 years on an in investment of $1, depending on your investor's expectations about timing of returns. Most investors in investment firms like venture capital, have very expectations for return on investment.
Because of these factors, most investment firms will only gather money from their investors when it is needed and not before. This decreases the amount of time between when the money is invested and the ultimate return, thus increasing the return on investment. Investment firms gather money from their investors via a process called a "capital call." They issues a notice to their investors telling them how much money they need and these investors, Limited Partners, wire the money to the investment firm. This process typically takes about 1-2 weeks.