The 'is bundle' attribute is used for merchant-defined bundles and it is required for all countries except Austria, Denmark, Belgium, Canada, Norway, Sweden, India, Mexico, Poland, Russia and Turkey.
First of all you need to know, that the default value of the 'is bundle' attribute is false. The price attribute for your bundle should contain the total price. You should use the attribute only if your bundles have a base item, because this item is the featured item of all the items included in your bundle.
For example a mobile phone with a contract must use the 'is bundle' attribute. The attributes google product category, item group id, product type, the unique product identifiers (brand, GTIN and MPN), adult and variant attributes should describe the base item of your bundle. All other attributes should describe your bundle.
For manufacturer-defined bundles you should not use the 'is bundle' attribute, because these bundles already have unique product identifiers. For all other bundles you should provide the unique product identifiers for the base item of your bundle. What’s the difference to multipacks? Bundles are several different items, that belong together, whereas multipacks are multiple items of the same kind packed together.