Here is a list of all namespaces with brief descriptions:

Ncxx14
Ndetail
NGU_VDBPointToolsInternal
Nhoudini_utils
Nllvm
Norc
Nnanovdb
Nio
Ntools
Nmorphology
Nutil
Nimpl
Nmorphology
Nopenvdb
Nv13_0
Nax
Nast
Ntokens
Ncodegen
Ncodegen_internal
Ninternal
Nmath
Ncompression
Ninternal
Nio
Nlogging
Nmath
Ninternal
Nmat3_internal
Npcg
Ninternal
Npoints
Nattribute_traits
Nfuture	A container for ABI=5 to help ease introduction of upcoming features
Nindex
Ninternal
Npca_internal
Npoint_attribute_internal
Npoint_move_internal
Npoint_rasterize_internal
Nrasterize_sdf_internal
Ntransfer_internal
Nstring
Ntools
Ncomposite
Nds
Ngridop
Nlocal_util
Nlstrack
Nlvlset
Nmorphology
Npoisson
NScheme
Nvalxform
Ntree
Niter
Nleafmgr
Ntypes_internal
Nutil
Nopenvdb_houdini
Nnode_info_text	Namespace to hold functionality for registering info text callbacks. Whenever getNodeSpecificInfoText() is called, the default info text is added to MMB output unless a valid callback has been registered for the grid type
NOSN
Nstd
Nunittest_util