What is a multi-token string?
Ok I misinterpreted it for a general concept about strings that I would not known, but it seems to actually be Elasticsearch specific Jargon:
By default, when processing fields mapped as strings, ElasticSearch parses them and tries to broke them into multiple tokens, and it seems to be the case for strings containing / or . As a consequence, those strings become "multi token strings". To avoid that, one need to edit the mappings of ElasticSearch and set the field as "not_analyzed", eg:
"my_field2": { "type": "string", "index": "not_analyzed" }