Linnworks allows you to create Packaging Groups consisting of Packaging Types which are used to control the size and weight of the items that can be dispatched within one single package. Packaging Groups can then be assigned to your orders automatically if your inventory items have weight, dimensions and a Default Packaging Group added in Product Details.
Packaging Groups and Types are created and edited via Shipping > Packaging Groups.
Follow the guidance steps here for creating Packaging Groups and Types.
The Prevalence option in the Packaging Groups screen is used for orders that have multiple items with different weight, dimensions and Default Packaging Groups. Linnworks will then choose and assign the Default Packaging Group which has the highest Prevalence assigned.