OP's phrasing of the answer:
Both of these resource types are located within a single zone, they have a zone field accordingly to specify where to provision them. Since a zone is located in a single region, specifying the requested zone for the resource is enough because it implicitly specifies the region as well. There is no option to specify the region for these resource types because that would be redundant along with specifying the zone, and specifying only the region would not be enough.
Original answer provided:
Both of the resources you linked have the zone tag, which is where instances and VM disks need to be located, as they are not region-wide. Zones are located within a region, and usually there are two or three zones for each region.
For example, taking the region us-west1, in this list you can see that it has the zones a, b and c, which when specified in the zone tag need to be written as us-west1-a, us-west1-b or us-west1-c.
Edit:
This example shows an example terraform configuration file, which creates two different Compute Engine VM instances in two different zones, located in two different regions:
provider "google" {
project="YOUR-PROJECT" # Project ID
region="europe-west2" # Default resource region
zone="europe-west2-b" # Default resource zone
}
/*
* Create instance in region Europe West 1, zone b
*/
resource "google_compute_instance" "europe_instance"{
name = "europe-instance-1"
machine_type = "n1-standard-1"
zone = "europe-west1-b"
boot_disk {
initialize_params {
image = "debian-cloud/debian-9"
}
}
network_interface {
network = "default"
}
}
/*
* Create instance in US West 1, zone c
*/
resource "google_compute_instance" "us_instance"{
name = "us-instance-2"
machine_type = "n1-standard-1"
zone = "us-west1-c"
boot_disk {
initialize_params {
image = "debian-cloud/debian-9"
}
}
network_interface {
network = "default"
}
}
