A listing group is a group of listings. Once you've created a listing group, you can add message rules that will only send to the listings in the group and/or create a turnover calendar that will show only the check-ins and check-outs of the listings in the group.
Creating listing groups helps save you time and simplifies your message rule management by removing the need to create the same message rule for multiple listings.
Listing groups DO NOT sync availability across listings that are on different channels. If you have the same property on different channels then you will need to link the listings for Host Tools to recognize them as the same property.