How can I add a special character (for example, trademark) to a DITA file?

The <tm> element is recommended where you can use it. For example:

<tm trademark="DB2" tmtype="tm" tmclass="ibm">DITA</tm>
			

The tmtype for registered trademark is "reg", and for service mark it is "service".

The Unicode character entities for common symbols are:

• trademark: &#x2122;
• registered trademark: &#x00AE;
• service mark: &#x2120;
• copyright symbol: &#x00A9;