bonds in subtraction operation - or problem of reading

In the curriculum of stage 1, year 1 appears (P.7):
 represent and use number bonds and related subtraction facts within 20
(1) What is the underlined phrase meaning?
(2) If I want to explain it in other words which words I will use?

here:
https://assets.publishing.service.g..._national_curriculum_-_Mathematics_220714.pdf

Page 7
I think the answer is in the example:

9 + 7 = 16; 16 – 7 = 9; 7 = 16 – 9

The "Number Bond" seems to suggest that there is some sort of "bond" between 7, 9, and 16 when considering only "+", "-", and "=" as operations. It seems to be a vague idea related to "closure". Why 9 - 7 = 2 doesn't suggest 2 is also in the bond isn't quite clear to me. The definition may be related to the three numbers defined by 1) Take two values. 2) Find their sum. 3) Consider the two subtractions from the greatest of the three. Additionally, they don't seem interested if the sum in #2 exceeds 20. This is quite arbitrary.

Frankly, I believe "Number Bond" to be an esoteric term created and utilized by the authors of the statutory document for purposes of the statutory document. I'm willing to be wrong.
 
In the curriculum of stage 1, year 1 appears (P.7):
 represent and use number bonds and related subtraction facts within 20
(1) What is the underlined phrase meaning?
(2) If I want to explain it in other words which words I will use?

here:
https://assets.publishing.service.g..._national_curriculum_-_Mathematics_220714.pdf

Page 7

I searched for a definition of "number bonds", and found many references. Here are three:

https://en.wikipedia.org/wiki/Number_bond

https://denisegaskins.com/2007/01/13/number-bonds-better-understanding/

https://www.peoriapublicschools.org/Page/22953

It looks like the drawing may be the main idea: something like a molecule's "bonds".

Clearly this isn't only a British idea, as I initially assumed, so maybe I should have heard of it! (I hadn't.) Your source shows me, on the other hand, that it isn't only American curriculum documents that fail to define their terms, something that has irritated me for years. Educators should know how to tell people what they mean.
 
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