As per Sec 99 of IPC 1860, There is no right of private defence against an act which does not reasonably cause the apprehension of death or of grievous hurt, if done, or attempted to be done.
Whereas As per Sec 94 of IPC 1860, Act to which a person is compelled by threats.—Except murder, and offences against the State punishable with death, nothing is an offence which is done by a person who is compelled to do it by threats, which, at the time of doing it, reasonably cause the apprehension that instant death to that person will otherwise be the consequence: Provided the person doing the act did not of his own accord, or from a reasonable apprehension of harm to himself short of instant death, place himself in the situation by which he became subject to such constraint.
Section 94 stands as a General Exception because a person is provoked by threats and in such a case a person is led by his actions due to the threats and its resulting consequences, A person takes an action as a resultant to threat.
Whereas in Sec 99, act which does not reasonably cause the apprehension of death or of grievous hurt, if done, or attempted to be done should not lead to any actions of a person as there are no causes of one bringing his private defense in action.
Therefore, Section 99 of IPC,1860 provides only an extent upto which the right of private defence can be used and Section 94 is provided as a general exception.