What are local subscriptions and server-side subscriptions in KMail? What is a subscription, what does it do?

Local subscriptions and server side subscriptions are very similar. When you subscribe to a folder, it is shown in the list of folders. If you are not subscribed to a folder, it will not be shown.

Server side subscriptions are stored by the server, and can be shared between various clients. Local subscriptions override server side subscriptions, and are only stored locally.

I am subscribed to all the folders on the server. Why can't I see some of them in KMail? They are visible in other email applications like Nylas Mail.