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 (data.exists("timestamp")) {
        const UniValue& timestamp = data["timestamp"];
        if (timestamp.isNum()) {
            return timestamp.get_int64();
        } else if (timestamp.isStr() && timestamp.get_str() == "now") {
            return now;
        }
        throw JSONRPCError(RPC_TYPE_ERROR, strprintf("Expected number or \"now\" timestamp value for key. got type %s", uvTypeName(timestamp.type())));
    }
    throw JSONRPCError(RPC_TYPE_ERROR, "Missing required timestamp field for key");
}
UniValue importmulti(const JSONRPCRequest& mainRequest)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(mainRequest);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, mainRequest.fHelp)) {
        return NullUniValue;
    }
    
    if (mainRequest.fHelp || mainRequest.params.size() < 1 || mainRequest.params.size() > 2)
        throw std::runtime_error(
            "importmulti \"requests\" ( \"options\" )\n\n"
            "Import addresses/scripts (with private or public keys, redeem script (P2SH)), rescanning all addresses in one-shot-only (rescan can be disabled via options). Requires a new wallet backup.\n\n"
            "Arguments:\n"
            "1. requests     (array, required) Data to be imported\n"
            "  [     (array of json objects)\n"
            "    {\n"
            "      \"scriptPubKey\": \"<script>\" | { \"address\":\"<address>\" }, (string / json, required) Type of scriptPubKey (string for script, json for address)\n"