Data entities in F&O can be exposed through OData protocol. You can also add OData actions by creating methods in X++ and decorating them with SysODataCollectionAttribute.

You can see an example in MssLeaveRequestDateEntity.getDirects():

[
    SysODataActionAttribute('getDirects', false),
    SysODataCollectionAttribute('return', Types::Record, 'MssLeaveRequestDateEntity')
]
public static List getDirects(
    str reportingPersonnelNumber,
    date fromDate,
    date toDate,
    LeaveRequestApprovalStatus leaveRequestStatus)
{
    List list = new List(Types::Record);
    ...
    return list;
}

It has parameters of several types and it returns a collection of entity records (additional metadata is provided in SysODataActionAttribute).

Now what if you want to add such a collection as an input parameter? It sounds straightforward, but if you do it, the action will completely disappear from the entity because it fails to generate.

I was able to get a confirmation that such a parameter type isn’t supported. It’s another case when event logs helped me. Namely, I found a warning in Application and Services Logs > Microsoft > Dynamics > AX-ODataService > Operational log. At the General tab, it said merely OData Metadata Build Warning, but the Details tab revealed much more:

All right, so it’s explicitly unsupported. Note that this is just about a collection of records; you can use collections of primitive types such as numbers and strings.

The fact that this kind of warning can be found in event logs may be the most important information in this blog post. It may help with debugging of other issues related to OData in F&O.