we calla a action in QTP by using RunAction statement

Syn: RunAction "Action1[called testname]", oneIteration, 
parameters

by using call to copy of action and call to existing action we can call the actions.

if we use call to copy of action, we can call nonreusable as well as reusable actions, and we can modify the script, and we can call many times and one action sheet will be added in datatable when ever you called (per every calling you get one action sheet)

if we used call to existing action we can call only reusable actions (in QTP 9.5 above all actions are reusable only) and the script is read only (we can not modify the script), and there will be only one action sheet if we call more than once also