Help with the definitions of fields in a VCF output by Strelka
What is the difference of ReadCount in INFO column and DP in FORMAT column?


I'm not super familiar with Strelka but I believe it follows the VCF spec, so DP in the FORMAT column should give the depth of coverage (ie read count) per sample. ReadCount in INFO is not in the VCF spec but based on the definition it sounds like the depth of coverage aggregated over all samples. 


  What is the difference of number of allele versus number of reads covering the variant position ?


I believe the "number of alleles used" refers to counts of each base that are used in the indel error modeling process as described in the preprint here: 

https://www.biorxiv.org/content/10.1101/192872v2.full

So it sounds like subsets of the number of reads covering the variant position.