what's the tab bar for in calendar?

I didn't find anything in the help menu either. I tried it the way other tabs work like in Safari. What OS version are you using? I am using macOS Sierra 10.12.6. As you see from the screen shot there are two tabs in calendar.

After some investigation you must have an external account like gmail or yahoo that have delegated caldav calendars it doesn't work with just iCloud account.

Ill try to explain more in another post if you don't understand

The tab is so you can open a calendar in a separate tab. The way it is done is to open calendar and then from the menu bar select window, and then select a calendar from the list towards the bottom of the menu, for example work. It will open an new window with only that calendar. You now have two calendar windows open. On the menu bar click on show the tab bar, it will then display in the tab area the name of the calendar you just opened. Then click on the name in the tab area and drag the new window onto the first one in the tab area of that window and you will make a tab with only that calendar on it. This would be useful if you have multiple calendars and it gets to full or confusing. It makes it so you can view each calendar separately or in groups as you like them. There may be another way but this is how I was able to figure it out. I hope this helps.

Mitch,

I never noticed that. You are correct in stating that it is "useless."

I suggest that you provide Calendar Feedback.

It is simply showing the name of the application .

Open calendar application > Take cursor on top menu bar & click on view > show Tab bar .