Hey Eurban,

Sadly there's not a way to make Layouts (nor context and defaults) specific for variants.

What you could do is make two different fields for the field you wish to make mandatory. Then one field is visible when viewing the main subject and when viewing the variant you show the other field and make it mandatory. Then you could make an expression field that shows the subject-field value and if it's empty it will show the variant-field value. This field you can show whenever the user is in navigation mode.

coalesce(table.columnValue, variant.columnValue)

It's a workaround but it does work.

Does this help you out?

Kind regards,
Mark Jongeling

Currently this is not possible. If you feel this is needed then please make a wish for the default to include  variant_id.