There are a number of concepts and principles of XML mark-up in general,
and DITA mark-up in particular.
Most DITA elements have an id attribute, which is used to uniquely identify the element when the element is re-used. In most cases, id attributes are optional, but an exception is the topic's root element, where it is mandatory.
A title is not the same as a heading. A topic, a figure, a table and a section should each have only one title.
Avoid hacking: using mark-up workarounds that either break the DITA language rules or break the semantic integrity.
Comments, draft-comment, and required-cleanup elements
Standard XML comment mark-up, and DITA specific draft-comment and required-cleanup elements, allow authors to leave notes, comments and unfinished edits that will not be included in the output when the document is published.
Use of class attribute
The class attribute is used in specialisation, and should not be set by a DITA author. The class attribute should not be confused with the outputclass attribute.
Similar element names
DITA includes semantic elements that have almost identical names to others, such as keyword and kwd. These similar elements are intended to be used in different authoring scenarios.