What's the difference between TAG_DONE and TAG_END in AmigaOS C code?

There is none - it's one and the same.

TAG_DONE and TAG_END are defined as identical clones in NDK2.0:

NDK2.0-4/include/utility/tagitem.h

#define TAG_DONE   (0L) /* terminates array of TagItems. ti_Data unused */
#define TAG_END TAG_DONE

NDK3.1 (and higher) has a slightly different definition of TAG_END, but it's still identical:

Includes\&Libs/include_h/utility/tagitem.h

#define TAG_DONE (0L) /* terminates array of TagItems. ti_Data unused */

#define TAG_END (0L) /* synonym for TAG_DONE */