Abbreviation: FEC = "Filter Effect Control" (Tamron)
The abbreviation FEC means "Filter Effect Control" and is seen on Tamron products.
Explanation from Tamron
FEC (Filter Effect Control) function enhances operational ease of Polarizing Filter use
The Filter Effect Control is designed to rotate the filter to the desired position while the hood is attached. This is accomplished by turning the FEC ring that in turn rotates the portion of the filter mount where the filter is mounted.