Conditional location selection with try_files. It may be useful in cases of A\B version of a site running on same domain name. You can do sticky cookie with this example.

conditional_location.conf

server {    
    set $versioned_location @version_one;
    
    if ( $cookie_condition ) {
        set $versioned_location @version_two;
    }
    
    if ( $cookie_site_version = "version_one" ) {
        set $versioned_location @version_one;
    }
    
    location / {
      try_files $uri $versioned_location;
    }
    
    # sticky cookie example
    location @version_one {
        add_header Set-Cookie site_version=version_one;
    }
    
    location @version_two {
    }
    
    # also if you have other locations configs, you need to rewrite them too 
    # for example for /assets location you may rewrite it to: 
    
    location /assets {
      try_files $uri $branched_location; # <----difference
      expires 1y;
      gzip_static on;
    }
    
}