The ad type record has a section called slots in the default layout for all AdServer types. 

This is the page layout for Google Ad Manager as an example:

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If you edit the record you can select one or more slots for the Master to define which size is allowed for this specific ad type. 

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The slot sizes in Advendio are mapped to the slot sizes in GAM/ Xandr.  If you are missing sizes in your picklist, please contact your Administrator.

Also, read which slot size to pick if you like to set up an 3.1.5.2.1 Interstitials (Google Ad Manager) .

• Master & Companion Slot - read the below section.

• Blocker Slot - - read the below section.

• Report Slot - 3.1.5.2.3 Reporting Slots

• Forecast Slot - 3.1.5.1.1 Forecast Mode (Google Ad Manager), 3.1.5.4 How to configure formats for your Xandr-related products (Ad Type)

1. Using Master and Companion Sizes (GAM)

The field (CompanionSize) can be used to control whether

• a master/companion roadblock or a 

• master/companion video line item 

is used when doing the Availability Check and Adserver push. 

View in GAM:

1.1. Examples

You can define the Slot-Sizes using one or both fields: "Companion Size" and "Master". The following combinations are examples of how you can set up your system:

1.1.a Only one Master Slot

1.1.b Multiple Master Slots

1.2. One Master with multiple Companions

1.3. Multiple Master Sizes with one Companion

2. Blocker slots 

Use Cases:

Your AdType is for an exclusive booking. You like to sell this exclusive ad on a page which does not use ALL slots on this page.  To realize this you need to block the "unused" slots from being sold by other campaigns. You, therefore, BLOCK them. 
Therefore select the other sizes which are not used as Blocker Slots to put in fake bookings to GAM for this order on all slots of the page:

All slot sizes will be submitted with priority 2 and sponsorship to GAM, as separate line items of the same order