Questions about how to use API gimp_layer_new()

[arya]

When I read the API document of gimp_layer_new(), I'm curious about what are explicit procedure calls in the below quote. 

Quote: Other attributes such as layer mask modes, and offsets should be set with explicit procedure calls.

So I looked into gimp source code for this API usage example and find they don't set the mask mode or offset before adding this layer to the image using gimp_image_insert_layer(). 

Code:

    layer = gimp_layer_new (image, _("Background"),
                             img_a->columns, img_a->rows,
                             image_type,
                             100,
                             gimp_image_get_default_new_layer_mode (image));
     gimp_image_insert_layer (image, layer, NULL, 0);

I have the same confusion for gimp_layer_group_new() but I find the code file above calls gimp_layer_set_mode(), gimp_layer_set_opacity() and other APIs as required in the quote below.

Quote:Attributes such as layer mode and opacity should be set with explicit procedure calls. 

So the questions are: what is the explicit meaning of the two quotes? which APIs should we call and how? Must we call these APIs?
Thank you very much!