It's the "Design size". Originally, it was meant to indicate the lead slug height -- in modern parlance, it includes its 'internal leading'. Any extra leading would be added manually (and I think original lead type was designed to be used without).

Nowadays, it doesn't really denote anything special measurement of the font. Especially it does not denote the x-height! Script fonts, for example, may have huge capitals (even 'extending outside the glyph box' -- an effect unable to mimick with lead!), in combination with very tiny lowercase characters.

The "12 pt" you enter is used to scale the font bounding box, and it's entirely up to the font designer where and how he put the characters inside this.

first thanks for the fast answer

but i am not sure i understand you very well
from what you said there is no true/solid 12pt size
because i can make a type that it's 12pt size will actually
be 10pt size of another type. so 12pt size can be all sorts of sizes

am i right?

Remember the plumb type blocks? It's the height of the block.

If you measure the printed type from baseline to baseline with zero leading (line spacing), then it should be 12 points with 12 point type.

I believe all the above statements are true.

In an average font, the cap height is about 70% of the point size, and the x-height is about 50% of the point size. But plenty of fonts diverge from the average, occasionally by quite a bit.

Cheers,

T

..i can make a type that it’s 12pt size will actually
be 10pt size of another type.

Correct. You cannot tell what font size was used just by measuring the text you see -- you have to have the exact same font for comparison.