Can someone explain when to use "relevance" and when "relevancy"?

Relevance is the more common form, according to grammarist.com:

Relevance vs. relevancy:

• There is no difference between relevance and relevancy. Though the latter is the older form, relevance is now preferred in all varieties of English. In this century, relevance is about ten times as common as relevancy in U.S. popular usage, and the gap is even wider in British, Australian, and Canadian sources.

Ngram relevance vs relevancy

Etymology:

• relevance (n.) 1733; see relevant + -ance. Related: Relevancy (1560s).

-ancy (suffix):

• word-forming element denoting quality or state, from Latin -antia, forming abstract nouns on past participle adjectives in -antem, appearing in English mostly in words borrowed directly from Latin (those passing through French usually have -ance or -ence; see -ance).