SSL vs BIO object in openSSL

Question

Can someone explain to me in the OPENSSL API when I should use SSLread () and when I need to switch to BIOread ().

I need to integrate openSSL code into my existing TLS-supported project so that it can be used by applications that need secure connections. I will use non-blocking sockets in my projects on how to do this.

I looked at some documents that, after creating and binding the sockets in the usual way, will use SSL_CTX and the existing socket identifier to create the SSL object. This SSL object performs the SSL_read () and SSL_write () operations. But I'm not sure when I should create a Bio object and use BIO_read () and BIO_write ()