Capture actions inject the volume of work into your model. They show that this is the first thing that takes place, causing the process you are modelling to take place.
Like other actions, they have a duration (how long the action takes to perform) and a frequency (how often the action happens) and in addition, you can determine the volume of output from the action.
In the properties for the selected Capture, you can see that 3 information assets are generated. The percentage can be altered to determine the flow of information through the rest of the model.
This makes it easy to simulate what happens to capacity if workload increases. You simply adjust the percentages of Capture output, and check your insights!