If I serve in a field mission and if my duty station becomes a family duty station, my family will join me. My question is why is the lump sum, in lieu of shipment, only $5,000 and not $15,000 which is the lump-sum at a dependent rate?

The amount of the relocation grant, which is in lieu of shipment of personal effects, is linked to the travel of the family members. As only family members are being authorized to travel to the duty station join the staff member, that portion of the relocation grant relating to family members is payable. The $5,000 portion of the relocation grant relating to family members is the difference between the single rate of the relocation grant payable when the staff member travels alone ($10,000) and the dependent rate when staff and at least one eligible family member travels together ($15,000).