To show sections to clients in the portal, go to Settings > Client Portal > Appearance and choose As a list grouped by sections or As a board grouped by sections.
Here's what the client experience looks like when sections are enabled as a board grouped by sections:
Create a section inside a project
Open a project and go to the Tasks tab
Click + New section
Name the section, or select a saved section template
Create a section template
Go to Settings > Data > Templates > Sections & Tasks
Enter a section title
Click + to add tasks under the section
Once added to a project, you can edit task due dates, assignees, tags, and other details without affecting the original template.