A photographer can take independent photos or think of them as a whole, a photographic series. That means a set of still pictures that have enough commonalities to make sense when we gather them together.
But putting several images together isn't enough to make a series. We only speak of a "series" when the images are part of a scheme and when they are related to each other. Drawing up a list of commonalities and complementary points thus enables determining its perimeter.
We can build up a series from photos already taken, individually, in totally different contexts. The commonalities are sometimes identified long after the shots have been taken, without having never influenced them.
A selection of photos for a slideshow or an exhibition is often made this way. To make an interesting subject presentation, we first identify a common thread, and thus a list of commonalities. Then, the appropriate photos are identified from all the available stock.
An exhibition of paintings often follows the same idea. The list of commonalities can be reduced to a limited number of factual criteria : same painter, same time, same subject (portraits, for example). Even if the sizes and colors change from one painting to another, a series exists.
However, it does not always work. When an exhibition is unsuccessful, it can be due to a list of commonalities that are too difficult to pin down for the public. Storytelling is also essential within the framework of an exhibition, especially with a complex topic.
You can also build a series after having established a list of commonalities (and complementary points) you wish. In my opinion, planning a series is the best way since you know exactly what you need to achieve it and you focus on your goal during the shooting. You don't take useless pictures. Some photographers even make a script and storyboard beforehand to know precisely the kind of photos they need.