The seals really don’t seem to have any meaning beyond generic “things in the sky”

The 8 cards each relate to an element or an alignment
-balance- fire
-spear- ice
-arrow- wind
-ewer- water
-spire- lightning
-bole- earth
-lady- light
-lord- darkness

And each have some form of representation of both of their members of the 12

The cards also when they swapped effects in ShB still retain their alignment effect as the umbral cards are the melee cards (balance/fire, arrow/wind and spear/ice) and the astral cards are the ranged cards

The seals meanwhile have zero connection to anything, the sun seal sits on the fire card but the moon symbol doesn’t sit on the ice card, the seals don’t necessarily sit next to each other in the months either (as spear and spire or ice and lightning share a seal) when the lighting months are in early summer while the ice months are in early winter

The seals are a gameplay mechanic that has no actual relevance to AST’s lore