Magento layout files contain a small set of XML tags that act as detailed instructions to the application on how to build a page, what to build it with and the behavior of each building block.
The Layout Handle is an identifier by which the application determines what to do with the updates nested by it.
If the name of the handle is <default> then the application knows that its nested updates must be loaded on almost all the pages of the store prior to loading page-specific layout.
If Magento finds handles other than <default>, it will assign the updates nested inside the handle to the according page specified by the handle. For
instance,
default
catalog_category_default (* also used in directory.xml)
catalog_category_layered (* also used in directory.xml)
catalog_product_compare_index
catalog_product_gallery
catalog_product_send
catalog_product_view (* also used in customeralert.xml, tag.xml)
catalog_seo_searchterm_popular
catalog_seo_sitemap_category
catalog_seo_sitemap_product
catalogsearch_advanced_form
catalogsearch_advanced_result
catalogsearch_result_index
catalogsearch_term_popular
checkout_cart_index (* also used in googlecheckout.xml, paypal.xml)
checkout_multishipping
checkout_multishipping_address_editaddress
checkout_multishipping_address_editbilling
checkout_multishipping_address_editshipping
checkout_multishipping_address_newbilling
checkout_multishipping_address_newshipping
checkout_multishipping_address_select
checkout_multishipping_address_selectbilling
checkout_multishipping_addresses
checkout_multishipping_billing
checkout_multishipping_customer_address
checkout_multishipping_login
checkout_multishipping_overview
checkout_multishipping_register
checkout_multishipping_shipping
checkout_multishipping_success
checkout_onepage_additional
checkout_onepage_index
checkout_onepage_progress
checkout_onepage_review
checkout_onepage_shippingmethod
checkout_onepage_success
cms_index_defaultindex
cms_index_defaultnoroute
customer_account (* also used in newsletter.xml, review.xml, sales.xml, wishlist.xml)
customer_account_create
customer_account_edit
customer_account_forgotpassword
customer_account_index (* also used in catalog.xml, newsletter.xml, poll.xml, review.xml, sales.xml, tag.xml, wishlist.xml)
customer_account_login
customer_address_form
customer_address_index
customer_logged_in
customer_logged_out
customer_account_logoutsuccess
giftmessage_index_edit
giftmessage_index_save
giftmessage_index_remove
newsletter_manage_index
paypal_express_review
paypal_express_review_details
review_customer_index
review_customer_view
review_product_list
review_product_view
reviews
sales_order_details
sales_order_history
sales_order_print
sales_order_view
shipping_tracking_ajax
tag_customer_index
tag_customer_edit
tag_customer_view
tag_list_index
wishlist_index_index