Shami, the Tension is that one role can be filled by multiple people. If the accountabilities of the role stay the same but you want address a specific area of the role then Focus comes in. Like in a Sales role all the Acccount managers have the same accountabilities but serve an other region. So what you can then do is give a Focus like North, South, West and East.
If you have the same role but people want to be able to modify this role to specific needs with special accountabilities for their area then it is wise to create an other role.
Hope this shines a light on it for you.