the relay docs contain fragment:

query rebelsrefetchquery {   node(id: "rmfjdglvbjox") {     id     ... on faction {       name     }   } }

what ... on faction on syntax mean?

there 2 uses of ... related fragments.

incorporating fragment reference

query foo {   user(id: 4) {     ...userfields   } }  fragment userfields on user {   name }

has effect of composing fields fragment embedding query:

query foo {   user(id: 4) {     name   } }

note fragments may compose other fragments.

inline fragments

these can used compose fields in type-dependent way. example:

query foo {   profile(id: $id) {     url     ... on user {       homeaddress     }     ... on business {       address     }   } }

in example, server determine whether return homeaddress or address field @ runtime, based on whether requested object user or business.