ATP is a bio-chemical (adenosine triphosphate) found in all living cells. Testing for ATP in water samples has become a popular means to supply information as to the relative microbial count, activity or cleanliness of the water sample. The amount of ATP is proportional to the number of living cells but is not always proportional to traditional CFU/ml measurements used for quantifing microbial populations. (larger cells show higher ATP values and this affects the final microbial count.)
When a cell dies, ATP production is stopped and the ATP which has been made is rapidly degraded. A measure of ATP is proportional to the number of living cells, including large and small cells. When there is no ATP detected, it is generally accepted that there are no more living cells.
Instuments that test for ATP are special luminometers that measure bioluminescence and express test results in relative light units (RLUs), so I am not sure as to your units being RALs unless they are being stated as some sort of relative absorption light (units) -- I'll do some checking on this.