Difference between TCAP Request Primitive and Indication Primitive [Telecom]

Hi,

Can anyone please explain to me the difference between [the] TCAP Request Primitive and [the] Indication Primitive?

I mean, when is [the] Request primitive used and when is [the] Indication primitive used?

----------------------------------------------------- Parameter | Primitive: TC-INVOKE |

----------------------------------------------------- | Request | Indication |

-----------------------------------------------------

thanks,

Rohit