Specifically, it adds an address into the wallet.

Easy to test now thanks to https://github.com/bitcoin/bitcoin/pull/14410
```
diff --git a/test/functional/wallet_importmulti.py b/test/functional/wallet_importmulti.py
index 9ba6860..bff37b2 100755
--- a/test/functional/wallet_importmulti.py
+++ b/test/functional/wallet_importmulti.py
@@ -95,6 +95,7 @@ class ImportMultiTest(BitcoinTestFramework):
         assert_equal(address_assert['iswatchonly'], True)
         assert_equal(address_assert['ismine'], False)
         assert_equal(address_assert['timestamp'], timestamp)
+        assert_equal(address_assert['ischange'], True)
 
         # ScriptPubKey + internal + label
         self.log.info("Should not allow a label to be specified when internal is true")
```

{
    if (!isRedeemScript && ::IsMine(*pwallet, script) == ISMINE_SPENDABLE) {
        throw JSONRPCError(RPC_WALLET_ERROR, "The wallet already contains the private key for this address or script");
    }
    pwallet->MarkDirty();
    if (!pwallet->HaveWatchOnly(script) && !pwallet->AddWatchOnly(script, 0 )) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error adding address to wallet");
    }
    if (isRedeemScript) {
        const CScriptID id(script);
        if (!pwallet->HaveCScript(id) && !pwallet->AddCScript(script)) {
            throw JSONRPCError(RPC_WALLET_ERROR, "Error adding p2sh redeemScript to wallet");
        }
        ImportAddress(pwallet, id, strLabel);
    } else {
        CTxDestination destination;
        if (ExtractDestination(script, destination)) {
            pwallet->SetAddressBook(destination, strLabel, "receive");
        }
    }
}
static void ImportAddress(CWallet* const pwallet, const CTxDestination& dest, const std::string& strLabel) EXCLUSIVE_LOCKS_REQUIRED(pwallet->cs_wallet)
{
    CScript script = GetScriptForDestination(dest);
    ImportScript(pwallet, script, strLabel, false);
    
    if (IsValidDestination(dest))
        pwallet->SetAddressBook(dest, strLabel, "receive");
}
UniValue importaddress(const JSONRPCRequest& request)