This article provides additional information when you are working with Calendar overlay(s) in SharePoint. You can refer this article if you wish to work with one Exchange and multiple SharePoint calendars in one SharePoint Calendar.
When you use a SharePoint Calendar and chose to add an Exchange Calendar as an overlay, you will see that it will always add the Exchange/Outlook calendar information for your own account and you cannot chose a particular Calendar e.g. for a meeting room.
Exchange Calendar overlay with a SharePoint calendar only works with the Exchange mailbox of the user who is logged on currently. It does not retrieve and display a particular Calendar e.g. for a meeting room or the calendar information for other users even if we explicitly specify a different user account in the Exchange Web Service url like https://servername/owa/USER@SOMEDOMAIN.COM/?cmd=contents&module=calendar
To add an Exchange calendar
In the Name and Type section, type the Calendar Name and select Exchange.
In the Calendar Overlay Settings section, type a Description for the calendar (optional), and change the default Color theme for the calendar (optional).
Click Find. Outlook Web Access URL and Exchange Web Service URL will be filled in automatically. If Find does not work in your environment, type the URLs manually
If you want this Exchange calendar to always display, select Always Show.
Click OK and then click OK again.
Below is a description of the fields that you see when you are creating or adding a new calendar:
Calendar Name: Any text you want to use.
Outlook Web Access URL: This link will be shown on the left pane with Pink as background color (configurable). This is the link to the Outlook WebAccess URL. For more information please contact your Exchange administrator.
Note: Selecting different colors for different calendars makes it easier to distinguish which calendar an event is from
Exchange Web Service URL: It is important to use the right URL pointing to the Exchange Web Service from where SharePoint gets the information. For more information please contact your Exchange administrator.
Once these fields are populated each user will see their own information in the Exchange calendar.
Exchange Calendar overlay only works with the Exchange mailbox of the user who is logged on currently. SharePoint knows the user who is logged in to the SharePoint Calendar page. With this information, SharePoint is able to contact the Exchange Web Service to retrieve the calendar data for that particular user only. For more information, see Work with multiple Exchange and SharePoint calendars in one SharePoint calendar
Note: By default, the calendar will only show limited (free/busy) information about the event for the logged on user. This setting is controlled by the permission level in the outlook calendar. You can change this from "Outlook | File | Options | Calendar | Free/Busy options" under "Calendar Options". The permission level is set to "free/busy time" by default. If you wish to show more details in the Exchange calendar in SharePoint, you can change the permission level to "Full Details".