Reference: https://www.cisco.com/c/en/us/td/docs/security/firepower/620/configuration/guide/fpmc-config- guide-v62/firepower_threat_defense_high_availability.html

Threat Intelligence Director cannot apply Block directly to a TAXII source because STIX data obtained through TAXII can contain complex indicators. A managed device takes enforcement action based on individual observables, such as an IP address, URL, domain, or file hash. After the TAXII content has been ingested, an administrator can configure Block on an individual observable or on a simple indicator containing one actionable observable. Complex indicators combine multiple observables and therefore cannot be blocked directly; the administrator must instead block the appropriate individual observables within them.
An element is the managed device that receives published intelligence, not an intelligence object on which this action is configured. Consequently, the two supported object types are an observable and a simple indicator. Cisco Threat Intelligence Director documentation