See also the List of all upgrades and the upgrade category.
Page 31, Column 3, Paragraph(s) 14, Core Rule Book
Upgrade: A Corporation card type that is installed in any sever and grants the Corporation various abilities.

Upgrades are a type of Corporation card that can be installed in servers for abilities and effects. Unlike agendas and assets, there can be any number of upgrades installed in any server. Also unlike agendas and assets, upgrades can be installed in central servers as well as remote servers. Upgrades installed in a central server are installed in the root of that server.

Page 9, Column 1, Paragraph(s) 4-6, Core Rule Book

Upgrades are improvements to a server that provide the Corporation with a wide variety of benefits and bonuses.

The Corporation installs upgrades in remote servers or the roots of central servers. Upgrades are the only card type that can be installed in the root of a central server. An upgrade is not active until it is rezzed by paying credits equal to its rez cost.

There is no limit to the number of upgrades that can be installed in a server. When the Runner accesses an upgrade, he can trash it by paying credits equal to its trash cost.

Page 13, Column 2, Paragraph(s) 1-2, Core Rule Book
An upgrade can be installed in any server. When an upgrade is installed in a central server, it is installed in the central server's root.

Unlike an agenda or asset, there is no limit to the number of upgrades the Corporation can install in any server, central or remote.

A region is a special subtype of upgrade limited to one per server.

FAQ 1.0.1Edit

From the FAQ. Also numbers 1.1.22, 1.2.9, 1.3.9, 1.4.9, and 1.5.9

Upgrades should be installed in such a way that the Runner cannot tell the difference between upgrades, assets, and agendas.

Page 9, Column 2, Paragraph(s) 3, FAQ
Where is an upgrade installed in a remote server?
An upgrade is installed in a remote server in the same position as an agenda or asset. The Runner should not be able to tell what type of card is installed in a remote server by its position. This is different than installing an upgrade in a central server, where it is always installed in the root.
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