Pooling from first dollar means that an entire claim is pooled and not counted towards the experience of the year. Many Extended Health Care plans include or have the option for out of province and/or out of country (OOC), emergency travel medical coverage. This coverage may have a separate premium or be included in the overall EHC premium.
If a claim occurs there could be much larger than a typical EHC claim so there is pooling to protect the plan from that experience. The pooling can be either "from 1st dollar" or not, perhaps from another threshold such as the "stop loss" threshold.