The 'Detail Comment' is to allow an additional comment to be included against either an individual file or a collection of files, being checked-in. This allows greater flexibility to include comments specific to certain files.
The 'Summary Comment' information is included with all files being checked-in. For example, you can summarise the purpose of an entire check-in action.
To activate the 'Detail Comment' button
1. Click on and highlight the file name in the "Name" column in the lower section of the dialog. You can multi-select files to include the same 'Detail Comment' on each file selected
2. The 'Detail Comment' button is now activated
3. Click on 'Detail Comment' button
4. Enter the comment, you can switch between 'Summary' and 'Detail' comments.
Also notice the 'Include Summary Comment' checkbox.
Selecting this will copy the 'Summary Comment' contents and pre-pend this onto the 'Detail Comment' content when saving to the 'Comment' field when the file is checked-in.