XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (287 page)

It's worth mentioning here that the rules for output of numbers are not the same as the rules for input. When a number is converted to a string, the results are determined by the casting rules given in Chapter 11. To summarize these:

• An
  xs:integer
  value is output as an integer, for example,
  42
  or
  -315
  
• An
  xs:decimal
  value with no fractional part is output as if it were an integer, with no decimal point. If it has a fractional part, it is output with at least one digit before and after the decimal point, and no other insignificant leading or trailing zeros. Examples of
  xs:decimal
  output are
  42
  ,
  -315
  ,
  18.6
  ,
  -0.0015
  .
  
• An
  xs:double
  or
  xs:float
  value that's within the range 1e-6 to 1e+6 (one millionth to one million, positive or negative) is output in the same way as a decimal. Outside this range, exponential notation is used, with one significant digit before the decimal point. Examples of
  xs:double
  output are
  42
  ,
  -315
  ,
  18.6
  ,
  -0.0015
  ,
  1.003e-12
  ,
  -8.752943e13
  .