I.R.I.S. Readiris 14 for Windows User Guide - Manual (Page 55)

Section 5: Selecting the Recognition Options
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
Click the tab Font type.

The character pitch is set to Automatic by default.

Click Fixed if all characters of the font have the same width.
This is often the case in old typewriter documents.

Click Proportional if the characters of the font have a different
width. Virtually all fonts in newspapers, magazines and books
are proportional.
Important: these document characteristics do not apply to Asian,
Hebrew or Arabic documents.