Use the iCal option when your calendar app is not in the list, for example Outlook for Windows or Mac, Thunderbird or another app that supports internet calendars. Subscribing adds the organizer's calendar to that app, and new and changed events then appear automatically.
On a Mac the iCal option does the same as the Apple option: it opens Apple Calendar. The screenshots below show that case; other apps look different but follow the same steps.
Steps
Open the calendar page you received from the organizer and click Add to Calendar. If the page asks for your name and email first, fill them in and continue.
Choose iCal. Your browser asks whether it may open your default calendar app. Allow it.
Your calendar app opens with the subscription link filled in and asks whether you want to subscribe. Confirm. On a Mac this is Apple Calendar: click Subscribe.
If the app asks for settings, set how often it checks for changes to every hour or shorter, then confirm. In Apple Calendar this is Auto-refresh, which is often set to Every week.
The calendar now appears in the calendar list of your app.
Good to know
Updates take time. Each app checks for changes on its own schedule, from every few minutes to once a day.
Read-only. You cannot edit events in a subscribed calendar.
Use Google, Apple, Office 365 or Outlook when you can. Those options open the right screen for that calendar directly.
Unsubscribe
Remove the calendar from the calendar list in your app.
Troubleshooting
Nothing happens when I choose iCal. No app on your computer is set up to open calendar links. Install or set a default calendar app, or use one of the online options.
The wrong app opens. Change the default calendar app in the settings of your computer, then choose iCal again.