The forward direction is defined as logically progressing from BOP x toward EOP x. The reverse direction is defined as logically progressing from EOP x toward BOP x. In serpentine devices, the logical forward or reverse direction has an alternating relationship to the physical motion of the medium.
The concept of being in some position means not being outside a defined region. The definition allows the position to be on the boundary of a defined region. When a volume is first mounted, the logical position is always at the beginning of the default data partition (BOP0). Whenever a volume is mounted and the medium motion is stopped, the position is in some partition. While moving between partitions, there is no stable position.
The concept of being at some position indicates being positioned to a logical or physical extremity of a partition. A sequential-access device may be positioned at beginning-of-medium, at BOP x, at end-of-data (EOD), at EOP x, or at end-of-medium (EOM), since these are stable positions at extremities of a partition.
The concept of being before some position indicates that there is some element (data block, filemark, setmark, or other defined point) which may be encountered when moving toward EOP x, if the proper commands are issued. Being positioned before a particular data block means that if the device receives a valid READ command, the data block is transferred to the initiator. This position may also be before EW x and EOP x, since these are defined points within any partition. However, if data has not been written to the end-of-partition, these points may not be accessible by the initiator.
The concept of being after some position indicates that there is some element (data block, filemark, setmark, or other defined point) on the BOP x side of the current position which may be encountered if the proper commands are issued. When a READ command for a single data block has been successfully executed, the logical position is after the transferred data block.