A 1-part Transfer transaction results in a single Transfer and a single Transfer Receipt) When a 1-part transfer is saved both a Transfer and Transfer Receipt transaction are are created. Prior to posting the transaction, the On Hand balances of the Inventory Lots remain unchanged but the Available balances are reduced from the sending Site and increased in the receiving Site. After the transaction is posted, the On Hand balances are removed from the sending Site and added to the receiving Site. 1-part transfers do not use an 'Ship Via' Site. 1 part transfers are most often used to record:

Transfers after they have occurred.
Transfers that take less than a day to complete (i.e., The 'STORAGE' Site is 2 miles away from the 'PRODUCTION' Site).
Transfers that are unlikely to create a variance (i.e., you are using Sites to identify locations within a site such as a Freezer or Staging area).
Changes in Lot Ownership