Page 2 of the Companies entry screen has a field called 'Mailsort SSC', standing for 'Standard Selection Code'. This will be filled in automatically when you enter a Postcode with a code in the format '21103'.
This represents the Mailsort system's 'Residue' and 'Direct' codes - ie a sorting order. This figure will only be modifiable if you are allocated Full or No Deletions access to the companion module.
Note that although the SSC field is on Page 2, the code refers to the primary (mailing) address on Page 1 of the Company record.
When entering Towns and/or Post Codes to Company records, the Postcode is checked, and the Mailsort SSC field is filled in with the relevant Post Office code. For Mailsort purposes, the Postcode checking is demanding to the extent that the first half of the code must exist in reality, not just in format.
The Postcode Outward/Full/None field is filled in automatically, also based on the Postcode, but using different rules to the SSC. F for Full Postcode means that both the Outward and the Inward (the first and second) parts of the Postcode are specified and are in a valid format. O for Outward means that only the first part is specified or that the second part is not valid. N for None means that nothing is specified or neither part is valid. Note that getting the message 'That Postcode is not valid' does not necessarily mean that you get an N - if the Outward part is valid and you tell it to 'Allow' the entry, then 'O' will be put in the Outward/Full/None field.
The Outward/Full/None field can be searched on in the More Choices window, which is very useful for quickly assessing the quality of the database's Postcodes. Mailsort requires 90% full postcoding.