A lead or shoot can have any number of tasks (also referred to as reminders). A task is a wonderful way to keep track of what you need to do for a lead or shot (i.e. send an email, post a photo on social media, call the client, etc.)
A task can have a deadline, which is how you'll most often use it. If you want a more generic task that isn't connected to the date, you can create a task without a due date.
How to add tasks to a lead or shoot
There are three ways to manually add tasks to a lead or shoot:
Go to the Lead > Overview page, select the Create Task option from the Add button just under the Shoot date in the right.
Go to the Lead > Dates & Tasks page and click the "Add New" button at the top of the page.
Apply a workflow to the lead or shoot, which is a great way to bulk-add tasks that you create in a workflow template.