A question on precipitation reactions

[ASidd]

My chem textbook says that in precipitation reactions you lower the pH of the reaction in order for the precipitate to form quantitatively.

It doesn't elaborate at all. What is the meaning of precipitate forming quantitatively?

 

[Borek]

Quantitatively - you can assume all was precipitated. Qualitatively - only some were precipitated.

First allows determination of an amount, second determination of the presence.