S emi controllable grates are generally less expensive than fully controllable ones. 

They are usually made from a metal plate which sits at the front of your fire, just below the grate, they are often not very effective slats that can be opened or closed.

Semi controllable grates are not fire cement sealed like controllable grates, the degree of control over the burn rate of your fuel is rather limited. 

Whereas the controllable grate allows your fuel to burn over a long period of time the semi controllable grate does not allow for a fire to stay alight unattended for several hours. 

In practice they operate rather as baskets, except they are made to fit into fires where a simple basket design is not suitable because of the shape of the hearth. 

As with controllable grates they are made to fit standard fireplace designs like the British inset range where the accuracy of the fit is less critical.

FAQ
Fireplaces and stoves ventilation requirements
Reinstating a fire place
The importance of fitting a stove correctly
What are fire baskets?