You may be familiar with the fact IBM Software Group has upped the minimum duration of software support from "3+2" to "5+3", or expressed in words: minimum 3 [now 5] years of support, with at least 2 [now 3] more years of support available at a charge. This better enables our clients to schedule upgrades at convenient times and not be forced to upgrade every few years.
An important extension of the IBM Software Support Lifecycle Policy is the statement that we will normally support N-2, meaning two feature releases prior to the latest release. This is an important part of enabling our clients to stay at the same release for a longer period. The "N" parameter is not necessarily the first digit in the release string, though. Each product may count differently, but I expect for Collaboration Solutions you'll see us counting each V.R (Version.Release or the first 2 digits of the release string) as feature releases. That should mean in the Notes/Domino and Sametime world, for example, that when we launch a release 9.0, we'll still be supporting releases 8.0 and 8.5 in parallel. However, if you step that backward once, you might think we should be supporting Sametime releases 7.5 and 8.0 for as long as 8.5 is the latest release. That would be true except that End of Support has already been announced for release 7.5 before publishing the N-2 policy, and that decision is grandfathered.
It's important to notice that the linked document is a policy, not a firm commitment. There will be variations, but in most cases we'll follow the standard policy of "5+3" and "N-2". Always check additional information specific to the product you're interested in. Note that this policy does not apply to software produced by the IBM Systems and Technology Group, such as operating systems. See also answers to Frequently Asked Questions linked directly within the Policy document above.