Abbreviation: FEC = "Filter Effect Control" (Tamron)

The abbreviations means "Filter Effect Control"

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.

See links

tamron-usa.com