The organization of Sequential and Relative files is different. Fixed nature is for Relative files. Fixed and variable nature is for Sequential files.
Then, how to choose the file organization? In this post, I have shared ideas for your reference.
A Sequential file that you can be accessed sequentially. That means you cannot supply the key. The length can be a maximum of Operating system supports.
The record can be either fixed or variable. If the incoming data is the same size always, then you can go for Sequential fixed-length.
Else, you can go for Sequential variable files
Record Can be Fixed or Variable
Relative files are different. They have a relative key. But, these are only fixed in nature. That means, it creates record for full length (as per definition) by padding spaces.
The maximum length of the record usually depends on your Operating system supports.
Relative file does not support variable length records.
Record is always Fixed
If you need fast access, you need to go for relative files.
Sequential files slow in response to fetch a record. Alternatively, relative files faster since they use a relative key.