On the tSlim the Insulin on Board tracks the full amount of any bolus (manually delivered by you and also automatic corrections if using Control IQ). It reduces over the time set for Insulin Duration, which on Control IQ is fixed at 5 hours.
If I remember correctly, IOB is adjusted downwards to take account of any basal suspends. I have to say I don’t find these reductions very helpful as it can artificially lower the apparent IOB if you have been active (and more insulin sensitive) and you’ve had an automatic basal suspend. It means mine can sometimes show 0u IOB, when I still have some meal insulin active within its 4-5hr duration of insulin action.