The auto-derived Route-Target (route-target import/export/both auto) is based on the Type 0 encoding format as described in IETF RFC 4364 section 4.2 (https://tools.ietf.org/html/rfc4364#section-4.2). IETF RFC 4364 section 4.2 describes the Route Distinguisher format and IETF RFC 4364 section 4.3.1 refers that it is desirable to use a similar format for the Route-Targets. The Type 0 encoding allows a 2-byte administrative field and a 4-byte numbering field. Within Cisco NX-OS, the auto derived Route-Target is constructed with the Autonomous System Number (ASN) as the 2-byte administrative field and the Service Identifier (VNI) for the 4-byte numbering field.
Examples of an auto derived Route-Target (RT):
IP-VRF within ASN 65001 and L3VNI 50001 - Route-Target 65001:50001
MAC-VRF within ASN 65001 and L2VNI 30001 - Route-Target 65001:30001
For Multi-AS environments, the Route-Targets must either be statically defined or rewritten to match the ASN portion of the Route-Targets.
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/7-x/command_references/configuration_commands/b_N9K_Config_Commands_703i7x/b_N9K_Config_Commands_703i7x_chapter_010010.html#wp4498893710