When defining an exchange setting, you can specify the type of file that will be used to exchange data. The following file type formats are supported for importing and exporting data:

• ASCII

• CSV (Comma Separated Values)

• Excel

• XML (Extensible Markup Language)

In addition, DIF, Excel, PDF, and Word are supported for exporting data. For more information about supported file types for exchange settings, see Sage Online.

Note  To import data from a file type that is not listed above, you must create an intermediate file (for example, a .txt file) using one of the supported formats; however, most applications provide a data exporting feature that supports one of the above file types.

Note  The ASCII text file type requires that each data field be positioned at particular character positions. All other file types must store each data field as a separate field or column."

So it may be as simple as the fact that your DIF file can be used to export, but not import. I've never tried to use that file type, so I can't speak more to that.

When I first saw your post I thought it might be the Date Format. Looking at another section of the Help Files, and I am reminded there are 2 different G/L Exchange types, Standard and Alternate:
"

Field Description	Alternate Exchange Format	Standard Exchange Format
Account Number	9 characters maximum (not including separators)	41 characters maximum (including separators)
Account Description	30 characters	50 characters
Transaction Date	8 characters Note Transaction dates must be formatted as MM/DD/YY.	10 characters Note Transaction dates must be formatted as YYYY-MM-DD.

"

If that doesn't answer your question, perhaps someone else can find the old KB article.