HTML - Dtd Explanation

Hi There,

I do not work normally with xhtml (doing C++ for quite a while) so my question might be rather basic. I am looking for explanation on the following (I spent a day reading the specs but could not figure it out). What is the meaning of (from xhtml1-transitional.dtd) the code snippet below, what is the logic to interpret it? I have problem understanding what the dot means in the entity declaration, so, for example if someone could explain the first line, like %special.extra refers to extra or special or to object/applet/img/map/iframe tags and mean that, and can appear in the xhtml code like that - it would be highly appreciated.

Also, as far as I understand, special is the composite of special.extra and special.basic. What is the reason those are separated? And, how do I know that those apply for text elements only (except for the comment which I suppose should not be parsed; or should it)?

Thanks

Code:

<!--=================== Text Elements ====================================-->

<!ENTITY % special.extra
   "object | applet | img | map | iframe">
	
<!ENTITY % special.basic
	"br | span | bdo">

<!ENTITY % special
   "%special.basic; | %special.extra;">