Based on what type of data UE is sending pucch format is decided. Eg : if SR has to send on pucch then Scheduling request resourceconfig from reconfig will give pucch - resource id which in turn maps to pucch resources in the reconfig. For Ack/nack UE may get pucch resource I'd from the dci.
There is a 3-bit parameter 'PUCCH Resource Indicator' inside DCI. This parameter points to one of the PUCCH-Config, which were already configured in UE's memory by RRC messages.
