SSL vs BIO object in openSSL

问题:

Can anyone please explain me in OPENSSL API's, when should I use SSLread() and when should I go for BIOread().

I need to integrate the openSSL code to my existing project to supported TLS , so that applications which needs secure connections will make use of it. I will be using non-blocking sockets in my projects , how to go about it.

I have gone through some documents which after creating and binding the sockets in the usual way will make use of SSL_CTX and the existing socket id to create a SSL object. With this SSL object , SSL_read() and SSL_write() operations are performed. But I am not sure when I should create Bio object and use BIO_read() and BIO_write()