<core:icon identifier="actions-brand-typo3" size="small" />

$iconRegistry->registerIcon( 'provider-svg', \TYPO3\CMS\Core\Imaging\IconProvider\SvgIconProvider::class, ['source' => 'EXT:icon_api/Resources/Public/Icons/provider_svg_icon.svg'] );

<core:icon identifier="provider-svg" size="small" />

$iconRegistry->registerIcon( 'provider-bitmap', \TYPO3\CMS\Core\Imaging\IconProvider\BitmapIconProvider::class, ['source' => 'EXT:icon_api/Resources/Public/Icons/provider_bitmap_icon.png'] );

<core:icon identifier="provider-bitmap" size="small" />

<core:icon identifier="actions-brand-typo3" size="default" /> <core:icon identifier="actions-brand-typo3" size="small" /> <core:icon identifier="actions-brand-typo3" size="medium" /> <core:icon identifier="actions-brand-typo3" size="large" /> <core:icon identifier="actions-brand-typo3" size="mega" />

<core:icon identifier="actions-brand-typo3" size="small" overlay="overlay-readonly" />

{icon}
{icon} {alternativeIcon}
small default large
{overlay}