I went to [the blast ftp database][1], there are 18 `nt` files, each is less than 800 MB, and for `refseq_genome` it has 83 files, most of which are larger than 800 MB, which means the `refseq_genome` is much larger than `nt` database. However, when I search the definition of `nt` on http://www.ncbi.nlm.nih.gov/BLAST/blastcgihelp.shtml, it says `nt` database include All GenBank + RefSeq Nucleotides + EMBL + DDBJ + PDB sequences (excluding HTGS0,1,2, EST, GSS, STS, PAT, WGS). No longer "non-redundant".

My question is:

1. In my understanding RefSeq Nucleotides should include `refseq_genome` and `refseq_rna`, so `refseq_genome` should be much smaller than `nt` database. why is `refseq_genome` alone is much larger than the whole `nt` database?
2. I tried one accession number `NZ_AARG01000001.1` from refseq bacteria genome, and blastn against `nt` and `refseq_genome` database. For `nt` case, it took a few seconds and got less than 10 hits. For `refseq_genome` database, it took more than 10 minutes and got more than 100 results (all the accession number began with NZ). Then I searched NZ and found NZ represent not completed project. So the difference between `nt` and `refseq_genome` is that nt doesn't include NZ records?

 [1]: ftp://ftp.ncbi.nlm.nih.gov/blast/db/
On ["RefSeq accession numbers and molecule types"][1] you will see that the RefSeq accessions with the prefix `NZ_` are from whole genome shotgun (WGS) projects. As such these are excluded from 'nt'. Looking through the other 'genomic' sections of RefSeq, many of these are from WGS projects and are thus also excluded.

From the [NCBI BLAST][2] pages 'nt' is currently defined as:

> Title:Nucleotide collection (nt)  
> Description:The nucleotide collection consists of GenBank+EMBL+DDBJ+PDB+RefSeq sequences, but excludes EST, STS, GSS, WGS, TSA, patent sequences as well as phase 0, 1, and 2 HTGS sequences. The database is partially non-redundant. In some cases identical sequences have been merged into one entry, while preserving the accession, GI, title and taxonomy information for each entry. Merged sequences include GenBank and RefSeq entries with identical sequences. Sequences added to the database since April, 2011 have also been merged with identical existing entries.  
> Molecule Type:mixed DNA  
> Update date:2014/07/24  
> Number of sequences:23840180

In contrast 'refseq_genomic' is defined as:

> Title:NCBI Genomic Reference Sequences  
>Molecule Type:mixed DNA  
> Update date:2014/07/23  
> Number of sequences:6733817  

Note the difference in the number of sequences. However the 'refseq_genomic' is much larger when you look at the number of bases: 435,293,002,525 vs. 62,649,172,490. This is due to 'refseq_genomic' including assembled contigs, and whole chromosome assemblies, which are excluded from 'nt'.

 [1]: http://www.ncbi.nlm.nih.gov/books/NBK21091/table/ch18.T.refseq_accession_numbers_and_mole/
 [2]: http://blast.ncbi.nlm.nih.gov/