This is the LDraw File Format Specification:https://www.ldraw.org/article/218.html
In the "Line Types" part,Line Type 1 part says:
Sub-files can be located in the LDRAW\PARTS sub-directory, the LDRAW\P sub-directory, the LDRAW\MODELS sub-directory, the current file's directory, a path relative to one of these directories, or a full path may be specified. Sub-parts are typically stored in the LDRAW\PARTS\S sub-directory and so are referenced as s\subpart.dat, while hi-res primitives are stored in the LDRAW\P\48 sub-directory and so referenced as 48\hires.dat
So, if u want to import a part with subfile into blender,be careful your subfile exists in the \ldraw\parts\s folder.