Difference between scaffolds.gff3 & top_level.gff3

[scyxr]

Dear all,

I'm recently trying to annotate my data using GFF file. However, I found two different ref.gff files for human in NCBT ftp site. see as follows:
########
ref_GRCh37.p13_scaffolds.gff3.gz
ref_GRCh37.p13_top_level.gff3.gz
alt_CHM1_1.1_scaffolds.gff3.gz
alt_CHM1_1.1_top_level.gff3.gz
alt_CRA_TCAGchr7v2_scaffolds.gff3.gz
alt_CRA_TCAGchr7v2_top_level.gff3.gz
alt_HuRef_scaffolds.gff3.gz
alt_HuRef_top_level.gff3.gz
################

those are the file names in GFF directory in the ftp site. I guess I should use ref*.gff3.gz files to do my annotation.
Can anyone tell me which one is useful for me? ref_GRCh37.p13_scaffolds.gff3.gz or ref_GRCh37.p13_top_level.gff3.gz or else?

What's the difference between these two?

Thanks.

xr