Facultative

But does the penny fare end here, said Mr. Nixon, at a merely facultative stop? Surely it ends rather at the station.

Source: wiktionary

The policy would be a “facultative” policy, whereby the insurers would first have assessed the risk involved in the particular shipment and decided to accept it.

Source: wiktionary

facultative feeder

Source: wiktionary

This hypothesis might be supported by the evidence that there are varieties of pathogenic fungi of which parasitism is in varying degrees, between obligate parasitic and facultative parasitic. The facultative parasite, which usually lives as a saprophyte but under some conditions can parasitize on a plant, seems to be a pathogen with the lowest parasitic adaptation.

Source: wiktionary