This is correct; however, the end-user of the class doesn't know. See how I documented it for JsonDecoder. I was asking in #symfony if somebody knows where "format" is specified, but it seems that it's up to the person who implements a specific decoder.

In other words: I opened the documentation for JsonDecode, and I was lost for quite a while because $format wasn't documented.

Do you suggest to change this to:

"The format parameter specifies which format the data is in; supported formats should be equal to the formats for which supportsDecoding returns true."