Single precision, called "float" in the C language family, and "real" or "real*4" in Fortran. This is a binary format that occupies 32 bits (4 bytes) and its significand has a precision of 24 bits (about 7 decimal digits).
Since 2^24 = 16777216 then there are your 7 digits, with a little over, so the calculation is probably telling you that it kept getting a difference up to the 8th digit.