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:
Any map segments loaded beyond the 2,025 segment limit, even if there is more room on the microSD card, will not be recognized or displayed by the unit.
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.