Racing Fuel Systems

What impacts the Idle AFR
Aside from mixture screws & float level is it the IFR or the transition slot restriction ??

Plus, any leakage around the t/blades & PCV if used.

There is fuel discharge from the t-slot even at curb idle unless the blades are closed more than they should be. But that will show up as other problems at light part throttle. Having the blades open too far will give too much fuel from the t-slots and make the mixture screw un- (or less-) responsive.

Getting the IFR/IAB/blade position (or t-slot area) combo wrong is when people jump to the 't-slot restrictor' as a cure-all, which in my opinion isn't the correct solution most of the time (there are exceptions).

OK so between IFR & transistion slot restriction the IFR has the most impact
Yes understand IAB’s and the throttle blade position / t- slot position must not be overexposed etc
Trying to understand why you would restrict the t - slot

Most of the time (not always!) a t-slot restrictor is a band-aid for a too-large IFR (or wrong IFR/IAB combo).

edit: or if it's a junky aftermarket baseplate with transfer slots too wide or long or in the wrong location.

T-slot restrictor. Aftermarket Holley carburetors have no means of adjusting/restricting the t-slot. Many OEM holley carbs did. Some mopars had an IFR and a t-slot restrictor/jet, often referred to as the first 3 circuit Holley. GM/Chevrolet sometimes used an air bleed in the lower venturi. Also some had an adjustable idle air bleed.
If you were to use an AFR gauge you would see that most/all aftermarket Holley carbs are too rich AFR when on the t-slot area. A lot of carb tuners disregard this.

Quadrajet , Carter and some OEM holley carbs were not.