Insertion loss, or the loss of signal that happens along the length of a fiber optic link, is expressed in dBs and should always be a positive number. But it can be negative (which isn’t a good thing).
Return loss, which measures the amount of light reflected back toward the source, is also expressed in dBs and is always a positive number. A high return loss is a good thing and usually results in low insertion loss.
Reflectance, which also measures reflection and is expressed in dB, is a negative number. High reflectance is not a good thing.