What is key_id arg in load_private_key() and load_public_key()

[Becky H]

Hello - 

Two openssl functions require *key_id as an argument.  What is this?

EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,
     UI_METHOD *ui_method, void *callback_data);

EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,
     UI_METHOD *ui_method, void *callback_data);

>From examples I've found on the internet, it appears to be some type of slot 
>id 
for a USB token. If this is true, how do I find the id?  I am using a Aladdin 
token 
on a Linux system.

Thank you.