"Legacy" 802.11 clients use 20MHz channels. To achieve higher data rates, 802.11n specifies the use of 40MHz wide channels. Channel bonding commonly uses two contiguous 20MHz channels to create a single 40MHz channel. While channel bonding can provide higher data rates, it will also result in fewer channels available for 2.4GHz or 5GHz devices.
In some high-density client environments where wireless saturation is high, having more available 20MHz channels to spread client usage provides better performance than having fewer 40MHz channels. In addition, not all regulatory domains support the use of a 40-MHz channel.
A wide channel is made up of a "primary" and "secondary" channel. The "primary" channel is used for 802.11n clients that only support 20MHz channel bandwidth (e.g. legacy clients). For clients that support wide channel capabilities, both primary and secondary channels can be used.
The location of the primary channel is specified when you configure channel selection. The primary channel is either the upper or lower half of the 40MHz channel. For example, if you are using wide channel in 5GHz and choose 36+, then channel 36 (lower half) would be the primary and the secondary would exist at channel 40 (upper half). Conversely, if you specified 40- you would be specifying channel 40 as the primary(upper half) and the secondary would be channel 36 (lower half).