<!ENTITY % special
   "A | IMG | OBJECT | BR | SCRIPT | MAP | Q | SUB | SUP | SPAN | BDO">

<!-- %inline; covers inline or "text-level" elements -->
<!ENTITY % inline "#PCDATA | %fontstyle; | %phrase; | %special; |
%formctrl;">

These have told us that SPAN is a member (indirectly) of %inline;

The following tells us that the content model _inside_ SPAN is also
%inline;

<!ELEMENT SPAN - - (%inline;)*         -- generic language/style
container -->
<!ATTLIST SPAN
  %attrs;                              -- %coreattrs, %i18n, %events --
  %reserved;                   -- reserved for possible future use --
  >

For P we see that it's %block;, but that its content model is %inline;

<!ENTITY % block
     "P | %heading; | %list; | %preformatted; | DL | DIV | NOSCRIPT |
      BLOCKQUOTE | FORM | HR | TABLE | FIELDSET | ADDRESS">

<!ELEMENT P - O (%inline;)*            -- paragraph -->
<!ATTLIST P
  %attrs;                              -- %coreattrs, %i18n, %events --
  >