Fixed Effects Interpetation
Interpretation of FE: $\delta_{st}$  is the mean rating in each sector in each time period.

Contrived Example:  Over time, public opinion of workers in the library sector improved and their rating r improved. On the other hand, those in the used-car sector became seen as less reliable and their rating r declined.  We capture this with $\delta_{st}$ .

Identification of B (intuition):  


x must vary within individuals i of the same sector s every time period. Otherwise $\delta_{st}$ will be co-linear. 
x must vary within individuals i along time periods t. Otherwise $\gamma_i$ will be co-linear.


Basically, $\gamma_i$ can be identified separately from $\beta$ because individuals change over time.  And $\delta_{st}$ can be identified separately from $\beta$ because they individuals are heterogeneous within each time period and sector.