When you model your structure and get forces on your members, those forces are a result of your assumed member stiffnesses.

You then design your members to resist those forces.

Sometimes those designs result in LARGER or SMALLER members than what you originally modeled.

This means that your actual structure is different than the one you modeled.

ACI is simply saying that if the difference between your modeled structure and your actual designed/specified members are more than 10% different, you
should re-model the structure with these different member sizes. This will result in different member forces and you can then re-design to the more
correct forces.

Theoretically you'd keep repeating this process until the model members and the final member sizes were within 10% of each other.

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It basically means that if your documentation (the size you want built) differed by more than 10% in the properties noted from that which was analysed, then you should be updating the analysis to the correct sizes.

I believe its meant to allow for some minor tweaking of member properties, say rationalising say concrete member sizes to the nearest 1/2" rather than 1/4" which analysis might have been based on. Make too many changes, or larger changes then you are obliged to revisit the analysis with these updates included.