The annotative property of a block is either set when creating the block definition or afterwards, in the Drawing Exlorer / Blocks dialog.
Annotative blocks can have an optional property to always match the orientation of a layout, independent from the rotation of the viewport content.
To make a block definition annotative
Open the Drawing Explorer / Blocks dialog.
If necessary, choose Detail View.
Annotative block definitions have the Annotative icon () displayed.
Click in the Annotative field of a block definition to toggle the annotative property on/off.