At some point of 17th century Catalan merchant ships started using white ensigns with a centered Coat of Arms of Barcelona or Catalonia, rather than the traditional yellow-and-red striped flags of earlier times (to let them look alike to other Spanish Empire flags perhaps?).
This particular design can be used for either merchant and privateering Catalan ships, except for the 1640-1653 period, when designs shortly changed.
Sources: Anonymous manuscript from 1670, reprinted in Amsterdam 1966 as «Flags of the World 1669-1670» | Francisco Manuel de Melo, «Historia de los movimientos, separación y guerra de Cataluña», Madrid 1645
Sizes: ensign W12 x L18 mm, mast flags W8 x L12 mm, jack W6 x L9 mm, pennant W4 x L48 mm.