The probability of alternative transcript

[dudududu12138]

Hello, I use Augustus to predict alternative transcripts. The result is in gff format. But I don't understand the 6th column. It looks like probabilities. But I noticed that the values of a gene is the sum of values of the transcripts. For example, transcripts g1.t1 and g1.t2 are 0.08 and 0.02 while gene g1 is 0.1. So what's the meaning of column 6? And do I need to filter the results according to column 6? If so, what's the proper threshold?
Here is my code:

singularity exec $augustus augustus --species=human \
        --alternatives-from-sampling=true \
        --sample=100 \
        --minexonintronprob=0.1 \
        --minmeanexonintronprob=0.4 \
        --maxtracks=-1 \
        chr21.fa >chr21.alt.gff

And the result is listed below:

Thank you very much!