diff --git a/engine/swarm/services.md b/engine/swarm/services.md
index 5f98a7f717..d7986fb983 100644
--- a/engine/swarm/services.md
+++ b/engine/swarm/services.md
@@ -501,7 +501,15 @@ placement of services on different nodes.
   limitations into account.
 
   Unlike constraints, placement preferences are best-effort, and a service will
-  not fail to deploy if no nodes can satisfy the preference.
+  not fail to deploy if no nodes can satisfy the preference. If you specify a
+  placement preference for a service, nodes that match that preference are
+  ranked higher when the swarm managers decide which nodes should run the
+  service tasks. Other factors, such as high availability of the service,
+  will also factor into which nodes are scheduled to run service tasks. For
+  example, if you have N nodes with the rack label (and then some others), and
+  your service is configured to run N+1 replicas, the +1 will be scheduled on a
+  node that doesn't already have the service on it if there is one, regardless
+  of whether that node has the `rack` label or not.
 
 
 #### Replicated or global services
@@ -509,7 +517,9 @@ placement of services on different nodes.
 Swarm mode has two types of services: replicated and global. For replicated
 services, you specify the number of replica tasks for the swarm manager to
 schedule onto available nodes. For global services, the scheduler places one
-task on each available node.
+task on each available node that meets the service's
+[placement constraints](#placement-constraints) and
+[resource requirements](#reserve-cpu-or-memory-for-a-service).
 
 You control the type of service using the `--mode` flag. If you don't specify a
 mode, the service defaults to `replicated`. For replicated services, you specify
@@ -568,8 +578,11 @@ the following example, the service only runs on nodes with the
 [label](engine/swarm/manage-nodes.md#add-or-remove-label-metadata)
 `region` set to `east`. If no appropriately-labelled nodes are available,
 deployment will fail. The `--constraint` flag uses an equality operator
-(`==` or `!=`). It is possible that all services will run on the same node, or
-each node will only run one replica, or that some nodes won't run any replicas.
+(`==` or `!=`). For replicated services, it is possible that all services will
+run on the same node, or each node will only run one replica, or that some nodes
+won't run any replicas. For global services, the service will run on every node
+that meets the placement constraint and any
+[resource requirements](#reserve-cpu-or-memory-for-a-service).
 
 ```bash
 $ docker service create \
@@ -584,12 +597,12 @@ file.
 
 If you specify multiple placement constraints, the service will only deploy onto
 nodes where they are all met. The following example limits the service to run on
-nodes with `region` set to `east` and where `type` is not set to `devel`:
+all nodes where `region` is set to `east` and `type` is not set to `devel`:
 
 ```bash
 $ docker service create \
   --name my-nginx \
-  --replicas 5 \
+  --global \
   --constraint region==east \
   --constraint type!=devel \
   nginx
@@ -615,6 +628,8 @@ Placement preferences are not strictly enforced. If no node has the label
 you specify in your preference, the service will be deployed as though the
 preference were not set.
 
+> Placement preferences are ignored for global services.
+
 The following example sets a preference to spread the deployment across nodes
 based on the value of the `datacenter` label. If some nodes have
 `datacenter=us-east` and others have `datacenter=us-west`, the service will be