What means the following message displayed on opening of a file?

In order to view the document correctly, you may have to change your font and/or script settings in the Font Dialog.

This question is already answer on the FAQ page. However, here is a more detailed explanation.

The file you open (probably HTML, XHTML or XML file) contains a character set or encoding declaration at top of the file which defines the code page / encoding of this file, for details see Character encodings in HTML and CSS, Declaring character encodings in HTML and Wikipedia - Character encoding.

Like browsers or XML interpreters UltraEdit and UEStudio take this information about how to interpret the bytes in this file as characters also into account. For information about bytes in a file versus characters displayed see power tip Unicode text and Unicode files in UltraEdit/UEStudio.

By default the configuration settings to auto-detect the used code page of an opened file are enabled at Advanced - Configuration - File Handling - Code Page Detection. Therefore UltraEdit/UEStudio detect and set the code page of the opened file as you can do manually for each file using View - Set Code Page which must be done when auto-detection is not enabled and the code page used by the opened file is not the same code page as defined in the Windows regional and language settings, i.e. code page of opened file is different to the system code page.

But just knowing the code page used for the content of an opened file is not always enough to get the bytes of this file also correct displayed as characters. Some editors always convert an ANSI file using code page ??? to Unicode on load and convert them back to ANSI with the correct code page on save. UltraEdit/UEStudio do not make such a conversion because that would be very problematic on editing large text files with hundreds or more MiBs to list here just one example. So UltraEdit/UEStudio keep an ANSI file on opening as is.

With View - Set Font you can define the font used for normal text editing which can be also a proportional font like Arial, and with View - Set HEX/Column Mode Font for viewing/editing in hex or column editing mode where it is required to use a non-proportional (fixed width) font like Courier New or Consolas. But the font you specify here must support the code page used by your files and you must have selected the correct code page to get the bytes of the opened file displayed correct as text characters.

I suppose that the font and/or the Script (= code page) of the font used for displaying the text of your files is not correct set. So please open View - Set Font and select an appropriate font with correct Script as needed for your files. Or the character set / encoding declarations are wrong in your files.

Of course you can also disable the auto-detection of the code page which makes file loading faster. But then you must always know and set by yourself the correct code page and font for those ANSI files not using the same code page as defined in the Windows regional and language settings or alternatively as set under Advanced - Set Code Page/Locale. The code page setting in menu Advanced should be changed only when you are for example a translator in USA (code page in the Windows settings set accordingly) who works mainly on Russian ANSI files with UltraEdit/UEStudio.

It is also possible to just disable this warning message by unchecking configuration setting Prompt to change font and script if different from default system code page at Advanced - Configuration - File Handling - Code Page Detection. That should be done when the file contents are always correct displayed by the used font because the character set / encoding information is wrong and you don't want to change that for some unknown reason.

Even more information about code page and font script can be found in topic Message about change of font and/or script settings in font dialog.