The role and relevance of any subcommittee for a specific project to develop, amend or withdraw ISSAIs, GUIDs or other pronouncements depend on the scope and purpose of the project and the area of expertise of the subcommittee. In many cases, there will be more than one subcommittee in relation to a particular project to develop on the content of IFPP.
In general, the subcommittees contribute to projects in the following ways:

Propose new projects in the Strategic Development Plan (SDP): If a subcommittee wishes to develop new pronouncements, it makes a suggestion in this regard for inclusion in the SDP for the framework of pronouncements.
Be responsible for a project: The subcommittees will set up a project group to carry out the project
Provide members to a project group: In the SDP 2017-2019, it is foreseen that most of the projects are driven through cross-cutting project groups which will include members drawn from the relevant subcommittees as well as (in some cases) from the Regions and IDI.
Provide the chair of the project group: The chair (or other key members) of a subcommittee may often be in a good position to also chair some of the projects to develop INTOSAI pronouncements within the subcommittee/work stream/working group's area of responsibility.
Review or comment on drafts as part of the quality assurance process (step 2 of due process).