When you're adding a topic, you can choose from a drop down list of topics topics or you can type one in freeform.
The drop down list is created and maintained by the System Administrator by clicking on Admin | Maintain Abbreviations | Note Default Topic in the Desktop Version.
You can use a topic from the drop down list by clicking on the down chevron or
you can type in a free form topic. Using the drop down list allows you to standardize the topics.
It is recommended that you standardize on the drop down lists so that your advisors and associates know which topic to use.