The Specialty is the field that your particular expertise falls under, when categorized in broader terms. This could be more than one field, depending on the interdisciplinary nature of your work.
Your specialty helps you connect with your colleagues with the same expertise.
To add your specialty:
1) Click on your name
2) Click on the Bio button on your profile window
3) On the bottom right corner of the Bio page, click the button next to Specialty
4) Select your Domain, Field and Specialty from the respective drop-down menu and Save.