Recommended coverage depth for ChIP Seq (H3K9me3 ) According to ENCODE recommendations

For broad-peak histone experiments, each replicate should have 45 million usable fragments

According to my understanding, a usable fragment means a uniquely mapped read / pair of reads.
However

H3K9me3 is an exception as it is enriched in repetitive regions of the genome. ... Tissues and primary cells should have 45 million total mapped reads per replicate.

I fail to grasp Encode's language.

Does &quot;total mapped reads&quot; mean uniquely mapped reads?
If so, why is 45 million &quot;an exception&quot; ? It is the same as the other broad peaks.

Any ideas where else can I look for an explanation?
In the initial description for &quot;usable fragments&quot;, ENCODE has the following (emphasis mine):

Usable fragment – A fragment is defined as the sequencing output corresponding to one location in the genome. If single-ended sequencing is performed, one read is considered a fragment. If paired-ended sequencing is performed, one pair of reads is considered a fragment. Fragments are considered usable if they pass the various filters in the ChIP-seq uniform processing pipelines. Used to evaluate ChIP-seq data.

Aligning reads to repetitive regions is difficult because they don't give unique alignments.
Reads from these regions will often have poor alignment scores and may not pass alignment QC checks.
Reads may also be counted as multi-mapped, and often methods call for uniquely mapped reads, which again, may not be true for a large number of reads from H3K9me3 ChIP-seq.
So for this particular histone mark, ENCODE loosens the definition for what you should count towards a minimum sequencing depth threshold.