This tag is documented in https://docs.oracle.com/cd/E19253-01/817-1984/chapter6-42444/index.html
Though I could not find some docs that describe it in detail, I found some code snippets.
/*
* Look up the string in the string table and get its offset. If
* this succeeds, then it is possible that there is a DT_NEEDED
* dynamic entry that references it.
*/
have_string = elfedit_sec_findstr(argstate->str.sec,
strpad_elt.dn_dyn.d_un.d_val, arg, &str_offset) != 0;
if (have_string) {
dyn = argstate->dynamic.data;
for (ndx = 0; ndx < numdyn; dyn++, ndx++) {
if (((dyn->d_tag == DT_NEEDED) ||
(dyn->d_tag == DT_USED)) &&
(dyn->d_un.d_val == str_offset))
goto done;
}
}case DT_USED:
case DT_INIT_ARRAY:
case DT_FINI_ARRAY:
if (do_dynamic)
{
if (entry->d_tag == DT_USED
&& VALID_DYNAMIC_NAME (entry->d_un.d_val))
{
char *name = GET_DYNAMIC_NAME (entry->d_un.d_val);
if (*name)
{
printf (_("Not needed object: [%s]\n"), name);
break;
}
}
print_vma (entry->d_un.d_val, PREFIX_HEX);
putchar ('\n');
}
break;http://web.mit.edu/freebsd/head/contrib/binutils/binutils/readelf.c
#define DT_USED 0x7ffffffe /* ignored - same as needed */
https://github.com/switchbrew/switch-tools/blob/master/src/elf_common.h