You choose it in the New plan window, where it is required and starts on Feature, and it appears as a small label in the plan's side panel, next to its state; the row itself shows the short code, the title, who proposed the plan and when it last changed, and the start of the description.
Next to the state filters, the All categories menu shows only the plans of one category, with how many each has in the chosen state; a category with no plans there is not offered. Plans written before categories existed, or by an agent that gave none, show no category label in their side panel and appear under All categories; SyncNow never invents a category, and editing such a plan shows No category, which you can change or leave as it is.
Agents read the category, and the environment a plan applies to (applies_to: STAGING, PRODUCTION or BOTH), with get_planning and search_planning, and set both with project_program. The server accepts only these twelve categories, also written as the app shows them, such as Bug fix, and refuses anything else with the list of values. In the app, Search the planning also finds the plans of a category by its name. See How do I find an item in Planning?.