By default, LastPass uses the 2nd level domain name to decide if a site's credentials should be autofilled.  For example, if you have saved a site with credentials for www.acme.com, then LastPass will consider all of the following URLs matches since they all have a 2nd level domain name of 'acme.com'.

www.acme.com

acme.com

login.acme.com

secure.login.acme.com 

You can also choose to have LastPass only offer the logins saved to a particular URL. You can do this by setting a URL Rule with exact host matching.  By setting this URL rule, LastPass will recognize any subdomains as part of the URL, and only offer the appropriate logins.

From the example above, if all the URLs shared the same login except secure.login.acme.com, you would want to  set a URL Rule with exact host matching for secure.login.acme.com.  This would ensure that only logins saved  to that URl are offered for autofill or autologin on secure.login.acme.com.