mutli-certificate in Wallet. How to specify which one to use?

Need clarify since I am new in the encrypted/secure in Oracle Wallet world

We have number of certificates are imported in oracle Wallet.  I was told, if we do not use wallet ( like if we use cURL ), the certificate you want to use

should be specify in the header .  But when I look up example on using Wallet, none of the example specify the certificate.  Am I missing anything

Here is the example code I look up

DECLARE

  HTTP_REQ UTL_HTTP.REQ;

  HTTP_RESP UTL_HTTP.RESP;

  URL_TEXT VARCHAR2(32767);

BEGIN

  DBMS_OUTPUT.ENABLE(1000000);

   UTL_HTTP.SET_WALLET('file:/u02/app/oracle/racdb/wallet', 'wallet_password');