Customizer: Unique identifier that distinguishes which image upload control is uploading an image

Thanks to the tipoff from @birgire in the comment, I was able to locate two things, an example of an implementation that includes context, here (also of note is that the Github gist has code for the very useful ability to access any image uploaded from this context previously!

https://gist.github.com/eduardozulian/4739075

/**
 * Example of inserting a section called "Branding" with a
 * context-based image uploader
 */
$wp_customize->add_section( 'my_branding', array(
    'title'    => __( 'Branding', '' ),
    'priority' => 30,
) );

$wp_customize->add_setting( 'my_logo', array(
    'capability'  => 'edit_theme_options'
) );

$wp_customize->add_control( new My_Customize_Image_Reloaded_Control( $wp_customize, 'my_logo', array(
    'label'     => __( 'Logo', '' ),
    'section'   => 'my_branding',
    'settings'  => 'my_logo',
    'context'   => 'my-custom-logo'
) ) );