Custom content elements are used to create non-native blocks within the messages editor. Essentially, building your very own custom content. For example, the blocks below, where to see a custom made tag and bullet list within a 2-column block.
To add your own custom content, simply drag a Custom element onto your message.
Now click on the block and you will see the element styling options in the area on the left. This is where you write your markup to build custom content. Note that you can also expand this text area by clicking on the expand icon marked in the red circle in the image below.
It is also important to understand the markup that wraps the custom HTML block in order to proporly write your own markup. To do that, click on the SPML Center icon (1), followed by HTML preview (2). This will display the HTML rendered (3) by the drag and drop editor.
Here, you notice that the text 'Custom HTML block' is wrapped inside 2 layers of <div>, but ultimately inside a <td>. This means that the custom HTML markup needs to start with a <table>, which follows the nested table pattern: table → tbody → tr → td → table...
If you're familiar with writing HTML for emails, then you should be able to create some pretty unique and interesting blocks by using the Custom HTML element.