I have had my sole-user machine for a while and I've just noticed that in the [386Enh]
section of the system.ini file there is a line which reads

EMMExclude=C000-CFFF,C000-CFFF

What does this mean and is it important or just a leftover from something? Is it something which is adversely affecting the performance of my machine??

(Win98SE)

This designates a block of extended memory that is being used for some purpose by Windows memory management [video memory perhaps?].
It's there to keep programs [other than the one it's reserved for] from writing to that section.
There should be a corresponding entry in the config.sys file.

   That command excludes a specific area of extended memory so that it is not used by anyone except the software which specifically requests it.  In the normal scheme of things, software picks out a section of memory which is available and uses it to carry out the thinking that it does.  If two software happen to use the same area or part of the same area, there are problems (thanks to memory management, that usually does not happen).  But some software cannot use just any area of memory and so that command is used to reserve the needed area for that specific software.
   Does that command affect the performance of your machine?  It can if you are running a lot of software.
   Is that command needed or a leftover?  I don't know.  It is not needed for standard Windows useage.  It may be used by a modem, video card, or something else.  If it is a leftover, removing that command from system.ini will improve the performance of your computer.  If it is needed, removing it may cause the software which needs that exclusion to malfunction.  I would think that the software would put the command back in when it sees that it is not there or it would warn you that it is missing.

Thanks, Ray and dlwolff0, for your helpful and erudite explanations.

The line in question in system.ini, i.e.   > EMMExclude=C000-CFFF,C000-CFFF <   was not reciprocated in config.sys so I decided to take a chance and I deleted it.

After many reboots I have not found any adverse effects (touch wood!). If anything I feel that perhaps the machine is a little more zingy. Or is it my imagination?

So thanks again, each.