what is #target_type in EntityAutocomplete?
The ID accepted from EntityAutocomplete is the ID of any entity defined from Drupal core, or third-party modules.

You don't normally need to have a list of all possible entity IDs, since you normally know which entity your code is going to autocomplete, but the list can be easily obtained using the following code.

print_r(array_keys(\Drupal::entityTypeManager()-&gt;getDefinitions()));


I ran drush ev "print_r(array_keys(\\Drupal::entityTypeManager()-&gt;getDefinitions()));" on my test site, and I got the following.

[0] =&gt; block
[1] =&gt; block_content
[2] =&gt; block_content_type
[3] =&gt; comment
[4] =&gt; comment_type
[5] =&gt; editor
[6] =&gt; field_config
[7] =&gt; field_storage_config
[8] =&gt; file
[9] =&gt; filter_format
[10] =&gt; image_style
[11] =&gt; configurable_language
[12] =&gt; language_content_settings
[13] =&gt; mollom_form
[14] =&gt; node
[15] =&gt; node_type
[16] =&gt; rdf_mapping
[17] =&gt; responsive_image_style
[18] =&gt; search_page
[19] =&gt; shortcut
[20] =&gt; shortcut_set
[21] =&gt; action
[22] =&gt; menu
[23] =&gt; taxonomy_term
[24] =&gt; taxonomy_vocabulary
[25] =&gt; tour
[26] =&gt; user_role
[27] =&gt; user
[28] =&gt; view
[29] =&gt; menu_link_content
[30] =&gt; date_format
[31] =&gt; entity_form_display
[32] =&gt; entity_form_mode
[33] =&gt; entity_view_display
[34] =&gt; entity_view_mode
[35] =&gt; base_field_override