PDCCH spans full coreset duration but only occupies subset of RBs in Coreset. UE doesn’t know the placement of PDCCH so it has to blind decode all the possible frequencies to see if PDCCH is scheduled or not inside coreset.
To reduce these blind decoding attempts network shares the possible sizes of PDCCH to UE and these sizes are called as aggregation levels and each aggregation level represented in CCE(Control channel element) terms.
1 REG = RB of one symbol duration
6 REG = 1 CCE
From the above image there are 2 coresets in each slot, so device scans PDCCH based on the aggregation levels configured which tells how many PDCCH are configured and of what size are they in. This makes device to take the decision how quickly to search PDCCH within a coreset.
