er words, a UE drops monitoring for PDCCH candidates of SS sets with the highest indices one by one until number of CCEs to be monitored by the UE in the slot is less than the limit. This is illustrated in Figure 6.27, where three SS sets are configured and the total number of CCEs to be monitored in slots is shown. In red slots, the limit of 48 is exceeded, and as a consequence the user-specific (USS) set S = 3 is not monitored by the UE. The situation when gNB configures more BDs and CCEs in a slot than the UE is capable of monitoring is called overbooking. Only a special cell (primary cell of a cell group) can be overbooked, and a gNB can never overbook CSS. Overbooking has been introduced to enable maximum usage of the UE's capabilities across the slots. The actual payload a PDCCH carries is called DCI, and several formats of DCI can be transmitted by a gNB, as listed in Table 6.5. The formats starting with 0 are used for uplink (UL) unicast scheduling, and formats starting with 1 are used for DL uni- cast scheduling. Formats starting with 2 are group-common formats. A UE is capable of monitoring for at most three unicast formats and at most four DCI formats in a slot. If a gNB configures a UE with more DCI formats to monitor, certain truncation and padding rules apply to align multiple DCI formats in size. Out of group-common 2_X DCI formats, the 2_0 slot format indicator (SFI) and 2_1 pre-emption indicator are the new features of NR compared to LTE. SFI in DCI format 2_0 indicates a slot combination from a set of preconfigured slot combinations of differ- ent slot formats. As shown in Figure 6.25, two slot combinations are indicated, each by one DCI2_0. The SFI = 0 has been preconfigured using slot formats: Format 0 (DL sym- bols only) Format 1 (UL symbols only) and Format 8 (13 flexible symbols + 1 UL symbol). The SFI = 1 contains slot Format 2 (flexible symbols only). A flexible symbol, if indicated by a gNB in DCI2_0, is no-transmit no-receive symbol unless gNB schedules DL or UL Table 6.5 Downlink Contr