Give me more details of some bits in infomask!!

here are the source codes from src/include/access/htup_details.h.
/*
* information stored in t_infomask:
*/
#define HEAP_HASNULL 0x0001 /* has null attribute(s) */
#define HEAP_HASVARWIDTH 0x0002 /* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL 0x0004 /* has external stored attribute(s) */
#define HEAP_HASOID_OLD 0x0008 /* has an object-id field */
#define HEAP_XMAX_KEYSHR_LOCK 0x0010 /* xmax is a key-shared locker */
#define HEAP_COMBOCID 0x0020 /* t_cid is a combo CID */
#define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */
#define HEAP_XMAX_LOCK_ONLY 0x0080 /* xmax, if valid, is only a locker */

And I can't understand these attrs:
1. external stored attribute(s), what is this? can you give a create statement to show me?
2. xmax is a key-shared locker/exclusive locker/only a locker, so how you use this? can you give me a scenario?
let me try to explain it:
if there is a txn is trying to read this heaptuple, the HEAP_XMAX_KEYSHR_LOCK bit will be set to 1.
if there is a txn is trying to delete/update this heaptuple, the HEAP_XMAX_EXCL_LOCK bit will be set to 1.
but for HEAP_XMAX_LOCK_ONLY, I can't understand.
And another thought is that these three bit can have only one to be set 1 at most.
3. t_cid is a combo CID? what's a CID? give me an example please.
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