The rstbox_logs table is used by EngageBox to track the box "open" events for each user. The Limit Impressions option relies on this table to determine whether and when a user has opened a specific box. The size of this table depends on the traffic the website gets and the total boxes published on the page. You can safely truncate (empty) that table whenever you wish. Another thing you may consider is lowering the maximum days that EngageBox uses to truncate/empty the log table. You can find this setting on Global Configuration > EngageBox > Advanced.