The Mark of Seed is required to enter the Battlefield for the next mission.
If you obtain the Mark of Seed, you will be unable to obtain the Azure Key until you enter the battlefield for the next mission.
If you obtain the Azure Key, you will be unable to obtain the Mark of Seed until you use it at the Treasure Coffer in the Tenshodo HQ and wait until the following Earth day (Japanese midnight).
When you obtain the Mark of Seed, you will no longer be under a time limit, the quest is considered 'complete' and you will be able to return to gather a group if you haven't already.
If you have completed the battlefield previously, wish to obtain an Azure Key, and select the Mark of Seed accidentally, this can be remedied. Enter the battlefield -- which consumes the Key Item -- and leave. You may start collecting a new set of Seed Afterglows immediately. Waiting for JP midnight is not required, in this case.
After getting the Mark of Seed, you are welcomed to warp out, then use the Fei'Yin Home Point #1 without fear of losing the key item.
Seed Afterglow Locations
On the first map (upper level), there is one Afterglow in each of the four "large" rooms.
These give you Red, Orange, Yellow and Green facets on the back of your hand.
Number 4 has moved. It is in small room at H-9/I-9, east of the ramp to lower level (east of the A).
On the second map (lower level), there is one Afterglow in the large main chamber, and three behind random Cermet Doors in the "clusters" of doors (one to the south, southwest, and west of the large main room). The Afterglows can be targeted through the doors and walls, making them easier to find.
These give you White, Cerulean, Silver and Gold facets on the back of your hand.
The final Afterglow is where the Shadow NMs spawn. Go up the tunnel in E-7 from the basement to get to the other side of the first map. The Afterglow can be in a few rooms. Check E-7/F-7, F-6/F-7, and F-7/F-8.
This will give you the Blue facet on the back of your hand.
It can also be in the 2 small rooms off the hallway there.