The "Level" of your license limits IO tags. IO Tags exchange data with the field automation devices (reads and writes). The IO Tag count is limited by the Hardkey and matching Software License file.
All IO Tags and All Block Parameters are summed to calculate the Tag count.
See also 184.108.40.206 Internal Tags limited by license, TagLimit, TTLIOTag, TTLTag, TTLCTag, AccTag, CalTag and 14.1.2 Connection Transfer Limits.