Reserve space or not?

When you want to specify that a certain piece of content should be displayed one some subslides but not on others, the first question should be what should happen on the subslides it is not displayed on. You could either want

• that it is completely not existing there, or
• that it is invisible but it still occupies the space it would need otherwise (see the docs of the #hide function)

The two different behaviours can be achieved using either #only or #uncover, respectively. The intuition behind it is that, in one case, content is only existing on some slides, and, in the other case, it is merely covered when not displayed.