Lump Sum pay items as the name suggest are based on a 'Lump Sum' amount allocated to a portion of work. Progress payments can be released for these pay items based on the level of completion.
Rate pay items are based on a 'Rate' of work completion. Payments for these can be claimed as units of work are completed.
Milestone pay items are based on a milestone in the contract or event that needs to be fully complete before the associated payment can be claimed by the contractor.