Post categories are a way to categorize your posts - you can think of them as tags, or really just categories, if that makes more sense to you.
Create a New Post Category
To create categories, go to Settings -> Post Categories, and create a new one.
You can assign a name to the category, pick a color that will identify this category for you, and optionally, write a short description.
How is this actually useful - let's say you post promotional posts once a day, some motivational posts twice a day, and links to articles 3 times a week.
You can then create three categories - Promotion, Motivation, and Articles, and you can apply these categories to your posts.
Apply Categories to Posts
You can apply categories by editing each post and selecting a category from the dropdown.
You can select the posts in your lists, click on Modify Posts and assign a category to the selected posts here:
Or you can apply a category before you add posts to your list by clicking on More Options and selecting the category before you click add to list.
Manage Posts with Categories
Categorized posts will have a colored box in top left corner - this color is the same as the one you selected when you created a category.
If you hover over the color, you'll see the name of your category appear.
Posts with categories will also be surrounded with a colored border that matches the color of the assigned category.
Other than just looking nice, categories actually make it easier for you to manage your posts.
For example, you can sort the posts in your list by post category.
In search, you can specify the category you're searching for, so you can see only the results from the selected category.
When you're searching for content in an existing list, you can specify the category of posts that you're looking for, and Postradamus will show you only the posts that were assigned this specific category.
But most importantly, you can set up schedules for categories.
To set up a specific schedule for your categorized posts, assign the category while you're adding a time to the schedule.
You can also click on existing times to update them and add categories to them.
So what will happen when you apply a schedule with categories to your list?
Postradamus will fill each scheduled time with one post. If you specified a category for a time block, Postradamus will schedule one post with that specified category. If you haven't specified any categories in a time block, Postradamus will schedule any post (categorized or not) for this time block.