Plugin defines many WordPress filters to make it easily extendable:

Filter tag Occurance Arguments
horizon_listing_type_supported In listing types configuration, submission supported [bool], post type [string]
horizon_listing_detail_sections In listing detail sections [array], post type [string]
horizon_listing_detail_section_root_dir In listing detail path to templates dir [string], section [string]
horizon_attribute_value In listing detail value [string], field [array]
horizon_mail_subject In notifications, claim and report listing emails subject [string], mail action [string], template arguments [array]
horizon_mail_body In notifications, claim listing emails body [string], mail action [string], template arguments [array]
horizon_billing_fields In payments, user profile billing fields [array]
horizon_invoices_types In invoice post type definition invoice types [array]
horizon_invoices_next_invoice_number When generating the very next invoice number next number [string]
horizon_invoices_tax_rate When calculating item base price tax rate [double], supplier vat number [string], customer vat number [string]
horizon_mail_actions_choices In definition of mail templates actions [array]
horizon_filter_fields In filter definition fields [array]
horizon_order_query In query filtering to set order query [object], params [array]
horizon_filter_query In query filtering to filter posts by ids ids [array], params[array]
horizon_currencies_current_currency_index When receiving current currency currency index [int]
horizon_currencies_current_currency_rate When receiving current currency rate currency rate [double]
horizon_package_durations In package system package durations [array]
horizon_payment_proceed_gateway In payment gateway definition proceed [bool], gateway [string]
horizon_payment_gateways In purchase form gateways [array]
horizon_claims_allowed_listing_post_types In listing detail allowed post types [array]
horizon_shop_allowed_listing_post_types In listing detail allowed post types [array]
horizon_filter_field_plugin_dir In filter form to set field plugin directory for template loader directory [string], template [string]
horizon_filter_sort_by_choices In filter form sorting choices [array]
horizon_submission_can_user_create In submission permission granted [bool], user id [int]
horizon_metabox_location_polygon_enabled In post type definition (and submission) enabled [bool], post type [string]
horizon_metabox_location_street_view_enabled In post type definition (and submission) enabled [bool], post type [string]
horizon_metabox_location_inside_view_enabled In post type definition (and submission) enabled [bool], post type [string]

Each filter can be applied in your code using standard WordPress way, using apply_filters() function:

apply_filters( $filter_tag, $arg1, $arg2 ... );

To hook on filter, use add_filter() WP function:

add_filter( $filter_tag, $callback, $priority = 10, $accepted_args = 1 );