The Sequence collection is comprised of the sequences entered into the gene, protein, or plasmid collections. Changes to the sequence will be reflected both in the sequence collection and in the corresponding gene/protein/plasmid.

When creating an entry in any of the above collections that includes a sequence it will be automatically added to the Sequence collection along with information of the source entry, type (nucleotide or amino acid), accession number, & organism of origin.

Additional details can be manually added by the user by customizing the collection.

When creating a plasmid though a GenBank file the sequence will show any and all annotations present in the file in the In the "Sequence & Map" tab on the sequence's page

You can locate a wide array of information by searching for a partial sequence via the global search bar. For example: all the mutations you keep of a specific gene.

Not turning this collection on will not affect the gene, plasmid or protein collections, but can serve as a way to distill the information in regards to a sequence without the added properties associated with its source. For example - antibiotic resistance is an important aspect for plasmids, but less important when examining the sequence itself.
it will still be displayed in the map, but there is no need for a field in the collection.

Case is not important when entering a sequence, and you can change a sequence to be either lower or upper case.

And finally - sequences can be edited in every way - by typing, cutting+pasting, etc.