TPSF12CX-DESIGN-CALC: How to understand the meaning of the tool

Part Number: TPSF12CX-DESIGN-CALC

Hello, 

I have some trouble with the design tool "TPSF12CX-DESIGN-CALC". 

1.Which picture should I use to determine the unstable frequency and how to determine it?

2.The column for unstable frequency in the table is in red. What does this mean?

3.How to understand the picture "AEF Gain for Attenuation Performance"?Why does magnitude have both positive and negative one?

Thanks.