Vertex Attributes, Vertex Arrays, and Buffer Objects - Praise for OpenGL ES 30 Programming Guide - page 145

glBufferData will reserve appropriate data storage based on the value of size .
The data argument can be a NULL value, indicating that the reserved data store
remains uninitialized. If data is a valid pointer, then the contents of data are
copied to the allocated data store. The contents of the buffer object data store
can be initialized or updated using the glBufferSubData command.

void glBufferSubData (GLenum target , GLintptr offset ,
GLsizeiptr size , const void * data )

target

can be set to any of the following targets:
GL_ARRAY_BUFFER
GL_ELEMENT_ARRAY_BUFFER
GL_COPY_READ_BUFFER
GL_COPY_WRITE_BUFFER
GL_PIXEL_PACK_BUFFER
GL_PIXEL_UNPACK_BUFFER
GL_TRANSFORM_FEEDBACK_BUFFER
GL_UNIFORM_BUFFER

offset

offset into the buffer data store and number of bytes of the

size

data store that is being modified

data pointer to the client data that need to be copied into the

buffer object data storage

After the buffer object data store has been initialized or updated using
glBufferData or glBufferSubData , the client data store is no longer
needed and can be released. For static geometry, applications can free the
client data store and reduce the overall system memory consumed by the
application. This might not be possible for dynamic geometry.

We now look at drawing primitives with and without buffer objects.
Example 6-5 describes drawing primitives with and without vertex buffer
objects. Notice that the code to set up the vertex attributes is very similar.
In this example, we use the same buffer object for all attributes of a vertex.
When a GL_ARRAY_BUFFER buffer object is used, the pointer argument
in glVertexAttribPointer changes from being a pointer to the actual
data to being an offset in bytes into the vertex buffer store allocated using
glBufferData . Similarly, if a valid GL_ELEMENT_ARRAY_BUFFER object
is used, the indices argument in glDrawElements changes from being
a pointer to the actual element indices to being an offset in bytes to the
element index buffer store allocated using glBufferData .