What is the use of イ. in this document?

Question

While reading this page, which is about the Bank of Japan, I came across イ.

日本銀行は、日本銀行当座預金のうち、ゼロ金利が適用されるマクロ加算残高 の算出に用いる基準比率(「補完当座預金制度基本要領」4.(3)イ.に定める 基準比率)について、次のとおり定めることとしました。

I am not sure about the meaning of it. Is it interest?

Answer 1

「イ」is part of the numbering format for legal documents and is used to for subsections.

The order is いろは順, and would go: イ、ロ、ハ、ニ、ホ、ヘ、ト... In English this would be "a, b, c..." or maybe "i, ii, iii..."

If you read the announcements correctly, is says:

...the benchmark ratio (as determined under 2.3.イ in "Principal Terms and Conditions of Complementary Deposit Facility" has been determined as follows.

So you would need to look up the specified article (2.3.イ) in the specified document "補完当座預金制度基本要領". (There is an English version of the document, and there it is listed as 2.3.a.

If you want to read up on the basic formatting of legal documents, Wikipedia has an article. This pdf also makes it easy to visualize.

Answer 2

It's common to use katakana as bullet points like this when subdividing documents. If you look at the text of the specific document in question, you'll see it has a section 4:

4．適用利率

It has a subsection 3:

（3）付利対象積み期間...

And that has a subsection イ:

イ．基準平均残高に別に定める一定比率（以下「基準比率」という。）を乗じた金額

I think it's also very common to use ㋐, ㋑ etc. For example from here:

❸ 他にある作用を与える。他に影響を及ぼす。
  ① 好ましくないことを相手に及ぼす。 「妻にはずいぶん苦労を－・けてきた」 「他人に迷惑を－・ける」
  ②
    ㋐ 人に対してある感情を持つ。「先輩に思いを－・ける」「犯人に情けを－・ける」「 …に疑いを－・ける」