A sequential PS File doesn’t have any partitions or members(0 members). So, it doesn’t have a directory to look upto. Thus, the space allocated to a directory, in terms of Directory Blocks for a Sequential PS File is 0.
On the other hand, for Partitioned Dataset(PDS), it can contain 1 or more members. So, it needs a directory to record the whereabouts(location) of its members, which are scattered throughout the memory randomly. You need to allocate space in terms of Directory Blocks to a Directory in Partitioned Dataset(PDS). Thus, Directory Blocks for a PDS should be some finite number, say 5 blocks, or 10 blocks.
Suppose you are creating a new File AGY0157.EMPLOYEE.DATA. You specify the Directory Blocks on the ISPF Menu 3.2 screen, as follows -
1. If you want the file AGY0157.EMPLOYEE.DATA to be a Physical Sequential File(PS), then fill the Directory Blocks field = 0.
![Image85[1]](http://lh6.ggpht.com/_sQvdFWqMlMg/S1F5qxkhREI/AAAAAAAAB5A/hDH6u286jXA/Image85%5B1%5D_thumb%5B3%5D.png?imgmax=800 "Image85[1]")
2. On the other hand, if you want the file AGY0157.EMPLOYEE.DATA to be a Partitioned Dataset(PDS), having 1 or many members, and a directory, specify Directory Blocks field = 1,2,3 or ...(any finite value) blocks. In the figure below, I have put Directory Blocks = 5 Blocks. ![Image86[1]](http://lh5.ggpht.com/_sQvdFWqMlMg/S1F5u8UQZkI/AAAAAAAAB5M/sT-m6NZJVqQ/Image86%5B1%5D_thumb%5B3%5D.png?imgmax=800 "Image86[1]")
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