If you see Limited Functionality Mode (LMF) on your Skype for Business 2015 client or hear of users seeing this message, it doesn't always mean your Skype pool is down. You could just have a routing group that is not available for whatever reason. That's not a good thing either, but let's focus on the good news: the fact that your Skype pool is still functioning.
To understand why it might be easy to confuse the two technical problems -- after all, they have the same end result, i.e. no service -- let's review what routing groups do in Skype for Business.
Each Skype for Business Front End server can contain many different routing groups, depending on how many users are assigned to a particular pool. The relationship is as follows: User > Routing Group > Front End Server. So each user is assigned to a particular routing group, and each routing group is assigned to a specific Front End server. Each time a user is enabled in Skype for Business they are automatically assigned to a unique routing group.
A front-end server can contain many different routing groups. Administrators have no say in which routing group a user is assigned and they have no say so in which Front End server a routing group is assigned to. Each routing group can have up to three replicas. A Skype's Routing Group Quorum also has three replicas. These replicas are considered routing group replicas for the end users.
Now that we understand a little more about routing groups, here's why they might throw up the Limited Functionality Mode for users. If a routing group loses two of its three replicas, and you're down to a single routing group, it'll push you into LMF mode.
For example: You have four Front End servers (we'll call them A, B, C, D). Your particular routing group is located on servers A, B, C. You lose server A. What remains are servers B, C and D. Since you lost A, that means you have two replicas left, on servers B and C. With this, you still have a routing group quorum and are functional from a user services point of view.
But let's say you lose server B. That which leaves you C and D for the pool. But your routing group -- which was located on A, B, and C -- is now down to a single routing group on server C. Even down to a single routing group member (for your routing group), your Skype pool will still be functioning, but you are in LMF mode.
Now you may be asking yourself, "If I have a single routing group quorum, how do I have a pool quorum?" Even though you have pool quorum you have just lost routing group quorum, which means your Skype client, now displays "Limited Functionality Mode." Skype can limp along with two Front End servers in the pool as a worst-case scenario, but ideally, we want to get back three Front End servers as soon as possible.
Moving forward, if you see LMF on a client confirm the state of the pool by running the following cmdlets:
Get-CsPoolUpgradeReadiness and Get-CsPoolFabric.
Get-CsPoolUpgradeReadiness - Validates how many functioning Front End servers are registered in the pool.
Get-CsPoolFabric - Validates the health of the fabric of the pool along with the replicas.