I came across a very special requirement from my customer site to add just one field for storing tax registration numbers. 
This such a simple requirement for any one of us worked in SAP for few years. Huuu ... don't stop just here . My customer was too picky ...
The list of requirements went on adding like they wanted this field be dependent on the country as this is a tax registration number for a given country. Means this field should not show up for other countries. This field should not be editable by any one in production system as they are very critical for legal reasons. This field labels need to have the custom label meant for each country. The number of these fields can keep increasing on time to time for each country as per tax legislations. However the values under this field is required only to be printed on our custom built invoices. The only place I felt that I had smooth let go was this registration number was not used in any functionality driven by reports or tax calculations etc delivered by SAP out of box solutions. Me lucky here.
In this context ... My Gurus what will you recommend? No matter whichever table we append with fields we can not be driven by country. It was not an option to add Z-fields for any table. In my SAP carreer this was the first time I came across such a requirement and was puzzling me to find the right place in ECC. After some research I found a way out to match the above requirements and wanted to share with you all ... Again unleash the power of SAP and proved it's SMART again.
After refering some of the OSS Notes found a easy go ... Made use of the Global Data for Company Code which we need to make use while defining company code for each country specific legal entity. Added the additional data parameters which needs no code changes or development effort .... isnt it cool? I am sharing you the steps I followed to add these fields.
Go To SE16 and Enter Table name as T100I (company code - parameter types). and Hit F5 to navigate to Table Insert page. Key in the values like parameter type, parameter length, ISO country code, Description. Then Save. One record inserted in T100I table.
Now we can see this parameter in OBY6 - additional data page. Save the value in the table.
Entering any value for this parameter will be stored in T100Z. This is the home of new parameter introduced just now.
To perform this activity we need to open the client for modification mode.
Thats for today, share if you have a better way to handle such requirement. Thank you My Seniour who was so particualr and made me to explore this option.