soft coal vs sea coal

soft coal and sea coal both are nouns.

Word	Noun	Adjective	Verb	Adverb
soft coal	Yes	No	No	No
sea coal	Yes	No	No	No

As nouns, sea coal is a hyponym of soft coal; that is, sea coal is a word with a more specific, narrower meaning than soft coal:

soft coal: rich in tarry hydrocarbons; burns readily with a smoky yellow flame
sea coal: pulverized bituminous coal; used as a foundry facing

Other hyponyms of soft coal include cannel coal.

soft coal (noun)	sea coal (noun)
rich in tarry hydrocarbons; burns readily with a smoky yellow flame	pulverized bituminous coal; used as a foundry facing