Good explanation ctdna positive below analytical range

Can you explain what "Positive Below Analytical Range" means on a Signatera test? This has happened twice and both time the values were under 1. There is a complicated explanation for this from Natera, however this is technically a positive test meaning that cancer was detected in the sample but at a lower level of detection than can be accurately quantified. Sometimes the amount of material being measured by a test is less than the ability of the test to detect it. This can often be the case when looking for individual cells or parts of cancer cells. The Signatera test is best used serially (every 3 months) to look for residual cancer - in some situations the detection of ctDNA moves above and below the threshold of detection possible reflecting the bodies immune systems effort to eradicate the cancer.