Picture a door with a name on it. Behind the door are people who know one subject very well, and the only thing they sell is what they know. You can look at that door and mean the business behind it, or look at what comes out of it and mean the paid advice itself.
Wiktionary records consultancy as consultant + -cy, or consult + -ancy. Endings like -cy and -ancy often name a trade or a role, which fits both readings: the business of consulting, and the consulting that business does.
本批資料的 Wiktionary 條目把 consultancy 拆成 consultant 加 -cy，或 consult 加 -ancy。-cy／-ancy 這類字尾常用來指一門行業或一種身分。這正好同時解釋了兩個意思：一個是「做這一行的公司」，一個是「這一行做的事」。
Wiktionary entry 'consultancy' (noun), etymology_text supplied in this packet: 'From consultant + -cy or consult + -ancy.' (source_ref kaikki:3f6f69a190036ae4:1210296761)
One branch
賣專業意見的生意
兩個意思只差在鏡頭對準哪裡：對準門後面的組織，就是公司；對準門裡送出來的東西，就是服務。文法會替你分：有 a、有 s 是公司，沒有就是服務。
noun · B2
A company whose business is giving expert advice to other companies or organisations about one subject, such as management, engineering, design or computers.
顧問公司