This personalization string is used to direct a subscriber to a web version of the email, allowing them to view the email online even if their email client does not support HTML. For more information on personalization strings, please see the documentation here: https://help.salesforce.com/articleView?id=mc_es_personalization_strings.htm&type=5.
The %%view_email_url%% personalization string is a system-generated link that allows subscribers to view a web page version of an email. It can be included as the href attribute in a 'view online' link in an email template to enable users to view the email in a browser if they have trouble viewing it in their email client. The %%view_as_webpage%% personalization string is deprecated and should not be used. The %%vawp%% personalization string is an alias for %%view_as_webpage%% and should not be used either. The %%view_online%% personalization string does not exist. Reference: https://help.salesforce.com/articleView?id=sf.mc_es_view_email_url_personalization_string.htm&type=5 https://help.salesforce.com/articleView?id=sf.mc_es_view_as_web_page_personalization_string.htm&type=5