default for \fB@IMAGEX_PROGNAME@ capture\fR, except when writing to standard
output (\fIWIMFILE\fR specified as "-"), and also for \fB@IMAGEX_PROGNAME@
append\fR, except when appending to a WIM that is already pipable.
+.TP
+\fB--delta-from\fR=\fIIMAGE\fR
+Only for \fB@IMAGEX_PROGNAME@ append\fR: declares that the new image being added
+from \fISOURCE\fR is mostly the same as the existing image \fIIMAGE\fR in
+\fIWIMFILE\fR, but captured at a later point in time, possibly with some
+modifications in the intervening time. This is designed to be used in
+incremental backups of the same filesystem or directory tree. \fIIMAGE\fR can
+be a 1-based index or name of an existing image in \fIWIMFILE\fR. It can also
+be a negative integer to index backwards into the images (e.g. -1 means the
+last existing image).
+.IP ""
+When this option is provided, the append of the new image will be optimized by
+not reading files that, based on metadata such as timestamps, appear not to have
+been modified since they were archived in the existing \fIIMAGE\fR. Barring
+manipulation of timestamps, this option only affects performance and does not
+change the resulting WIM file.
.SH NOTES
\fB@IMAGEX_PROGNAME@ append\fR does not support appending an image to a split WIM.
.PP