«إذا» (idhā) commonly introduces a future event treated as expected or viewed as an occasion that will arise. Compared with «إن», it normally gives greater prominence to the anticipated occurrence.
«إذا جاء زيد فأكرمه»
When Zayd comes, honor him.
The instruction is linked to an arrival envisaged as a practical occasion, rather than to a merely open hypothetical event. This does not mean the particle itself proves that arrival will actually take place in the external world.
The useful contrast is between leaving fulfillment open with «إن جاء» and foregrounding the expected moment with «إذا جاء».