Suspicious tissue I believe is when a radiologist sees something, in my case it was coarse heterogeneous calcifications. A radiologist then labels them on a Birad scale of zero to 6. 6 being definite malignancy 5 highly suspicious and so on down the line. What I understand is that anything below a 4 is not necessarily reported. 4 = suspicious. Within the 4 is 4a, 4b, 4c. So at least at my healthcare facility they biopsy for a Birad score of 4.
I believe there are several other characteristics that influence the “suspicious” label, e.g. morphology, placement, are they linear or clustered.
This is why they have radiologists highly trained and experienced looking at mammograms and breast MRIs.
Disclaimer: the above is my interpretation of journal articles I have read.