Also \startchapter[title=mytitle1] ... \stopchapter Foo \startchapter[title=mytitle2] ... \stopchapter
From a structural point of view, Foo is outside the chapters
\chapter{mytitle1}
... Foo \chapter{mytitle2} ...
From a structural point of view, Foo is inside chapter "mytitle1"
Of course \startchapter[title=mytitle1] ... Foo \stopchapter% \startchapter[title=mytitle2]
... \stopchapter
is equivalent to \chapter{mytitle1} ... Foo \chapter{mytitle2} ...
So \start<section> \stop<section> can emulate \<section> but the converse is not true (at least from the p.o.w. of the structure of the document:
and anyway you can always write your own macros to change the things as you like )