XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (285 page)

That is to say:

• A sequence of one or more digits, with no decimal point or
  e
  or
  E
  , is interpreted as an integer literal. For example,
  0
  ,
  23
  , and
  0034
  are all integer literals.
  
• A sequence of one or more digits, with a decimal point among the digits or at the beginning or end, is interpreted as a decimal literal. Examples are
  1.50
  ,
  .001
  , and
  3.
  .