What does 'horizontal extension' mean in Unicode proposal docs?

[jk at koremail.com]

CJK Unified Ideographs are dealt with primarily by the IRG (ISO/IEC 
JTC1/SC2/WG2/IRG Ideographic Research Group) so their website is a good 
place to find out more https://appsrv.cse.cuhk.edu.hk/~irg/index.htm .

On 2023-11-14 07:47, Harriet Riddle via Unicode wrote:
> CJK Unified Ideograph blocks show more than one reference glyph for a
> lot of the characters.
> 
> Horizontal extension is adding an additional reference glyph to an
> existing already-allocated codepoint.
> 
> Vertical extension is allocating new codepoints.
> 
> The references to directions make more sense when you think about the
> distinctive layout of the code charts for the CJK Unified Ideograph
> blocks.
> 
> —Har.
> 
> On 13 Nov 2023 23:12, Sławomir Osipiuk via Unicode
> <unicode at corp.unicode.org> wrote:
> 
>> The phrase 'horizontal extension' appears in many proposals related
>> to CJK
>> characters, but isn't defined, and trying to intuit a definition
>> from
>> context is a bit risky.
>> 
>> What does this phrase mean formally, and what would the obvious
>> contrast
>> 'vertical extension' mean?