The RelatedToAllPackages field is one of the set of RelatedTo fields which can be used to search the "RelatedTo" (Destination) end of a relationship.
When querying relationships this allows you to specify the ID(s) of any related entities. ID's of entities are GUID's, so usage of this value normally looks like this:
This is a field that mostly exists for use in the future by API developers, for end-users, making use of the Destination or RelatedTo fields is recommended, as you can express your query in terms of names etc.
Return everything related to the entity with the unique Id 'c1dfd9b1-addc-4c76-af0b-9f8e00fea5d4':
Return everything directly related to the entity with the unique Id 'c1dfd9b1-addc-4c76-af0b-9f8e00fea5d4' (depth 1):