One of the options in the Match contents drop down list is Checksum & Size
This page explains what a checksum is.
A checksum is simply a number that is calculated from all the bytes in the file. A very simple example of a checksum would be to add together all the bytes. This is a true checksum in the original sense of the name, although simply adding the bytes of a file is rarely used to calculate a checksum these days. One of the deficiencies of this technique is that it's not very good at distinguishing between similar but different files. For example, two files with the same bytes but in a different order would have the same checksum.
SpaceMan 99 uses a more sophisticated algorithm called CRC-32.
CRC stands for Cyclic Redundancy Check, and 32 refers to the fact that the checksum calculated has 32 bits. This means that the checksum will be in the range 0 to 4,294,967,295.
Once the checksum has been calculated for two files, if the checksums are different it is guaranteed that the data in the files is different. However, if the checksums are the same it is not guaranteed that the files are the same. This is because many different files will have the same checksum. For a 32 bit checksum, the chances that two files with the same checksum are different is 1 in 4,294,967,296.