A sub-bundle is defined as:
An aggregation of project activities within a bundle having the characteristics that all project activities within a sub-bundle belong to the same type (EB 21, Annex 21, paragraph 4).
Therefore, a sub-bundle is a subset of project activities within a bundle, where all projects in the sub-bundle are of the same project type.
The provision for project activities in a bundle to be divided into a series of sub-bundles is set out in EB 21, Annex 21:
Project activities within a bundle can be arranged in one or more sub-bundles, with each project activit[y] retaining it[s] distinctive characteristics. Such characteristics include its: technology/measure; location; application of simplified baseline methodology. Project activities within a sub-bundle belong to the same type. The sum of the output capacity of project activities within a sub-bundle shall not exceed the maximum output capacity limit for its type (EB 21, Annex 21, paragraph 3).
Bundling, as defined in EB 21, Annex 21, involves:
Bringing together of several small-scale CDM project activities, to form a single CDM project activity or portfolio without the loss of distinctive characteristics of each project activity (EB 21, Annex 21, paragraph 3).
The advantage of bundling is that bundled projects can obtain a single validation report and a single certification report for the entire bundle.