Seals pre U14: Any chest that drops base items. Very low drop rate. No correlation between specific items and specific chests (e.g. the Seal of the Envenomed Blade could drop in any of these chests).

Shards pre and post U14: Any chest that doesn't drop base items plus any end chest. Again no correlation.

Yes I hate this system. Should have all shards in Spinner and seals split, 6 in LOD end chest and 10 in SOTO end chest.