A past participle represents a verb that has been completed but is mentioned in the present voice. Because the representatives are being spoken about in the active voice, but the meetings have already happened, a past participle is used. The past tense of 'to hold' is 'held', but adding the 'have' in front of 'held' turns it into a past participle.
"Hold" is present tense
"Holding" is present participle