The Sub-Sample Information box is designed to contain sub-sample information.
A sub-sample is a contiguous range of bytes of a sample. The specific
definition of a sub-sample shall be supplied for a given coding system (e.g.
for ISO/IEC 14496-10, Advanced Video Coding). In the absence of such a
specific definition, this box shall not be applied to samples using that
coding system.
The table is sparsely coded; the table identifies which samples have
sub-sample structure by recording the difference in sample-number between
each entry. The first entry in the table records the sample number of the
first sample having sub-sample information.
author  Alexander Simm

The sample delta for each entry is an integer that specifies the sample
number of the sample having sub-sample structure. It is coded as the
difference between the desired sample number, and the sample number
indicated in the previous entry. If the current entry is the first entry,
the value indicates the sample number of the first sample having
sub-sample information, that is, the value is the difference between the
sample number and zero.
return  the sample deltas for all entries

The subsample size is an integer that specifies the size, in bytes, of a
specific sub-sample in a specific entry.
return  the sizes of all subsamples

The subsample priority is an integer specifying the degradation priority
for a specific sub-sample in a specific entry. Higher values indicate
sub-samples which are important to, and have a greater impact on, the
decoded quality.
return  all subsample priorities

If true, the sub-sample is required to decode the current sample, while
false means the sub-sample is not required to decode the current sample
but may be used for enhancements, e.g., the sub-sample consists of
supplemental enhancement information (SEI) messages.
return  a list of flags indicating if a specific subsample is discardable