The point size expresses the em (height of the em-square) which is higher than the total height from the highest ascender to the lowest descender.
Edit: And then there is Leading; you may see that in the Character palette/panel.
Thanks for your reply. I don't totally understand it though. What is an em-square? Is it the "box" around the text, meaning the top of it being the highest point of the highest letter in the set and the bottom being the lowest of the lowest? Like the top of an 'h' to the bottom of a 'j'?
Also, if the requirement this document must meet is for the text to be 5mm in size can I go by the font illustrator applies when I select 5mm? What would be the standard: the em-square measurement (if the above is right), the measurement of a capital, non-descending letter, a lowercase letter like 'c'?
That is a strange request I've never come across in my years in this business. I'd be curious to know why someone wants to specifiy type in such a way. Body type is normally measured in point size, not millimetres. Ask them why they are specifying mm and not points.
Sounds like it could be from someone who thinks they know what they're talking about, but actually don't (I've come across quite a few of them in time)
Is it the "box" around the text, meaning the top of it being the highest point of the highest letter in the set and the bottom being the lowest of the lowest? Like the top of an 'h' to the bottom of a 'j'?
Yes, with a little adition in height: The em is the basic height holding any letter and then a bit (and was the side length of the basic square holding an M); it was defined in the good old days of manual typesetting, and sounds like an M. You may read a quick introcuction here: http://en.wikipedia.org/wiki/Em_%28typography%29
If you have more than one line of Type (I know you may not, in this case), the leading is also crucial (another term from the days when lead was handled by many). A quick introduction may be found here, too: http://en.wikipedia.org/wiki/Leading
I believe they are talking about cap/letter height, but maybe you should ask them, and refer to the terms that they ought to know, as KC points out. Or maybe they have only just laid down their rapidographs and stencils.
Is it the "box" around the text, meaning the top of it being the highest point of the highest letter in the set and the bottom being the lowest of the lowest?
No, it is not. The em square is a reference unit that is defined by the font designer. The actual placement of the letter inside this box is arbitrary in either direction, meaning that the letter often never gets near any edge of the box. This has to do with typographic things that affect kerning, hinting, special glyphs, ligatures and all that and how to "make it look nice". It also affects the perceived actual size of a glyph and therefore the density/ grey level of text. Anyway, I can only chime in with the others - your requirements for 5mm text sounds quite contradictory. That would only be relevant for graphical text as used in signage or logos and that is best done by converting the type to outlines and resize them to the required dimensions.