DYNSLT: Use this file-level keyword to indicate that the SELECTION and omission TESTS specified in the file (using select/omit specifications) are done at PROCESSING time. This keyword specifies dynamic select/omit rather than access path select/omit.
Only a Physical file contains data. Logical files just contain pointers for processing the data in a specific sequence and/or subset of data.
When a record is added/changed/deleted in a physical file, the system has to put a lock on it and then go out to CHECK and update any logical files before it can release the lock. While the amount of time to do this is hardly measurable, it can be significant if massive batch UPDATES are done to a file. 
If you are using the Select/Omit option in your DDS and using DYNSLT, the above process does not need to be done at record update time. 
Where this is beneficial is when your selection process includes almost all the records in the file. Now when you process using DYNSLT, it will read all the records in the file, but do the selection of which records to process at that time rather than having done all the maintenance ahead of time.

DYNSLT: Use this file-level keyword to indicate that the selection and omission tests specified in the file (using select/omit specifications) are done at processing time. This keyword specifies dynamic select/omit rather than access path select/omit.
Only a Physical file contains data. Logical files just contain pointers for processing the data in a specific sequence and/or subset of data.
When a record is added/changed/deleted in a physical file, the system has to put a lock on it and then go out to check and update any logical files before it can release the lock. While the amount of time to do this is hardly measurable, it can be significant if massive batch updates are done to a file. 
If you are using the Select/Omit option in your DDS and using DYNSLT, the above process does not need to be done at record update time. 
Where this is beneficial is when your selection process includes almost all the records in the file. Now when you process using DYNSLT, it will read all the records in the file, but do the selection of which records to process at that time rather than having done all the maintenance ahead of time.