Fragmentation
The process of partitioning an MSDU or an MMPDU into smaller MAC level frames, MPDUs, is called fragmentation. The MAC may fragment and reassemble individually addressed MSDUs or MMPDUs. The fragmentation and defragmentation mechanisms allow for fragment retransmission.

The length of each fragment shall be an equal number of octets for all fragments except the last, which may be smaller. The length of a fragment shall never be larger than dot11FragmentationThreshold unless security encapsulation is invoked for the MPDU. If security encapsulation is active for the MPDU, then the MPDU shall be expanded by the encapsulation overhead and this may result in a fragment larger than dot11FragmentationThreshold.

Under below conditions an MSDU not be fragmented even its length exceed the fragmentation threshold.
1. An MSDU transmitted under HT-immediate or HT-delayed Block Ack agreement
2. An MSDU or MMPDU transmitted within an AMPDU.
3. Group addressed MSDUs or MMPDUs

If a fragment requires retransmission, its frame body content and length shall remain fixed for the lifetime of the MSDU or MMPDU at that STA. Each fragment shall contain a Sequence Control field, which is comprised of a sequence number and fragment number. When a STA is transmitting an MSDU or MMPDU, the sequence number shall remain the same for all fragments of that MSDU or MMPDU.

The fragments shall be sent in order of lowest fragment number to highest fragment number, where the fragment number value starts at 0, and increases by 1 for each successive fragment. The Frame Control field also contains a bit, the More Fragments bit, that is equal to 0 to indicate the last (or only) fragment of the MSDU or MMPDU. Below diagram (page 223 – CWAP Study Guide) shows a MSDU that has been fragmented to 4 fragments.

Defragmentation
Each fragment contains information to allow the complete MSDU or MMPDU to be reassembled from its constituent fragments. The header of each fragment contains the following information that is used by the destination STA to reassemble the MSDU or MMPDU:
— Frame type
— Address of the sender, obtained from the Address2 field
— Destination address
— Sequence Control field: This field allows the destination STA to check that all incoming fragments belong to the same MSDU or MMPDU, and the sequence in which the fragments should be reassembled. The sequence number within the Sequence Control field remains the same for all fragments of an MSDU or MMPDU, while the fragment number within the Sequence Control field increments for each fragment.
— More Fragments indicator: Indicates to the destination STA that this is not the last fragment of the MSDU or MMPDU. Only the last or sole fragment of the MSDU or MMPDU shall have this bit set to 0. All other fragments of the MSDU or MMPDU shall have this bit set to 1.

References
1. CWAP Official Study Guide – Chapter 6
2. IEEE 802.11-2012 Standard.