Sometimes it's helpful to understand what a "master clip" is in Adobe-speak.
That means it is the metadata reference holder for media used on a sequence. So as a Master Clip, it exists as a stand-alone bit of media, with its own data in the project file. Including any effects that are applied say in a sequence but done using the Master Clip tab ... which are then applied across all instances of that clip in the projects.
A subclip exists as a linked bit of the larger clip ... the full metadata is still with the larger clip, but Premiere 'recognizes' that sub-clip as a segment that can be used by itself. But the master clip for it is still the clip it was taken from.
Neil