There are several products within the Outdoor and Fitness families that can accept microSD cards. The primary purpose of the microSD card is to store detailed mapping such as the street mapping City Navigator NT or the topographical mapping Topo 24K.
Garmin mode devices pertaining to this FAQ are the following devices:
There are limitations to the size of microSD card used, the number of downloadable maps that can be loaded and the number of detailed mapping segments that can be recognized by a device.
If at the most recent device software version, these limitations are:
If 2,025 detailed mapping segments are loaded to a microSD card, but the file does not reach 4GB worth of data, the device will not show any more detailed mapping than what is provided by the mapping segments.
See FAQ article, "How do I know what the latest firmware is for my GPS unit?" for instructions on how to properly update the device's software.
City Navigator: http://www.garmin.com/us/maps/citynavigatornt
Topo 24K: http://www.garmin.com/us/maps/topo