A method by which a first device performs wireless communication, and a device for supporting same. The method includes receiving, from a second device, first sidelink control information (SCI) including scheduling information about a physical sidelink shared channel (PSSCH) through a physical sidelink control channel (PSCCH), the first SCI including information related to priority, information related to frequency resource allocation, information related to time resource allocation, information related to a demodulation reference signal (DMRS) pattern, and information related to a modulation and coding scheme (MCS); receiving, from the second device, assistance information including information related to first resources and information related to a service type through the PSSCH; generating a medium access control protocol data unit (MAC PDU); selecting a sidelink (SL) resource from among the first resources based on the MAC PDU being related to the service type; and transmitting the MAC PDU based on the SL resource.