Macos 解释fst_posmode，F_peoposmode和F_VOLPOSMODE之间的区别？

头文件：

/* Position Modes (fst_posmode) for F_PREALLOCATE */

#define F_PEOFPOSMODE 3         /* Make it past all of the SEEK pos modes so that */
                    /* we can keep them in sync should we desire */ 
#define F_VOLPOSMODE    4       /* specify volume starting postion */

手册页：

The position modes (fst_posmode) for the F_PREALLOCATE command indicate how to use the offset field. 

The modes are as follows:
     F_PEOFPOSMODE   Allocate from the physical end of file.
     F_VOLPOSMODE    Allocate from the volume offset.

医生很难理解