It's also worth noting that you can use read depth to infer copy number, given a sufficient number of reads. On average, you'll get 1.5x more reads from a triploid region than you will from the rest of the diploid genome.
Sequencing depth represents the (often average) number of nucleotides contributing to a portion of an assembly.
On a genome basis, it means that, on average, each base has been sequenced a certain number of times (10X, 20X...).
For a specific nucleotide, it represents the number of sequences that added information about that nucleotide.
Such depth varies quite a lot depending on the genomic region. In consequence, an average sequencing depth of 30X leaves a lot of small portions of a genome unsequenced while other receive a lot more sequences.
I think, the depth means the coverage of the Sequencing technology only,
Coverage = (total number of bases generated) / (size of genome sequenced).
So a 30x coverage means, on an average each base has been read by 30 sequences. And the distribution in not always uniform. Some of the sequences may be covered more and some may be very less, so usually the coverage means an average value.