# Reserved capacity fleet properties
<a name="fleets.reserved-capacity-fleets"></a>

A reserved capacity fleet contains the following properties. For more information about reserved capacity fleets, see [Run builds on reserved capacity fleets](fleets.md). 

**Operating system**  
The operating system. The following operating systems are available:  
+ Amazon Linux
+ macOS
+ Windows Server 2019
+ Windows Server 2022

**Architecture**  
The processor architecture. The following architectures are available:  
+ x86\$164
+ Arm64

**Environment type**  
The environment types available when **Amazon Linux** is selected. The following environment types are available:  
+ Linux EC2
+ Linux GPU

**Compute instance type**  
The compute configurations for fleet instances.    
**Guided selection**  
Specify different compute types by selecting vCPU, memory and disk space settings. For information about compute type availability by region, see [About reserved capacity environment types](build-env-ref-compute-types.md#environment-reserved-capacity.types).  
**Custom instance**  
Manually specify the desired instance type.

**Capacity**  
The initial number of machines allocated to the fleet, which defines the number of builds that can run in parallel.

**Overflow behavior**  
Defines the behavior when the number of builds exceeds the fleet capacity.    
**On-demand**  
Overflow builds run on CodeBuild on-demand.  
If you choose to set your overflow behavior to on-demand while creating a VPC-connected fleet, make sure that you add the required VPC permissions to your project service role. For more information, see [Example policy statement to allow CodeBuild access to AWS services required to create a VPC network interface](auth-and-access-control-iam-identity-based-access-control.md#customer-managed-policies-example-create-vpc-network-interface).
If you choose to set your overflow behavior to on-demand, note that overflow builds will be billed separately, similar to on-demand Amazon EC2. For more information, see [https://aws.amazon.com/codebuild/pricing/](https://aws.amazon.com/codebuild/pricing/).  
**Queue**  
Build runs are placed in a queue until a machine is available. This limits additional costs because no additional machines are allocated.

**Amazon Machine Images (AMI)**  
The Amazon Machine Image (AMI) properties for your fleet. The following properties are supported by CodeBuild:      
[\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/codebuild/latest/userguide/fleets.reserved-capacity-fleets.html)

**Additional configuration**    
**VPC - optional**  
The VPC that your CodeBuild fleet will access. For more information, see [Use AWS CodeBuild with Amazon Virtual Private Cloud](vpc-support.md).  
If a fleet override is specified when calling the StartBuild API, CodeBuild will ignore the project VPC configuration.  
**Subnets**  
The VPC subnets that CodeBuild uses to set up your VPC configuration. Note that reserved capacity fleets support only one subnet in a single Availablity Zone. Also, ensure that your subnets include a NAT gateway.  
**Security groups**  
The VPC security groups that CodeBuild uses with your VPC. Ensure that your security groups allow outbound connections.  
**Fleet Service Role**  
Defines the service role for your fleet from an existing service role in your account.  
**Define proxy configurations - optional**  
Proxy configurations that apply network access control to your reserved capacity instances. For more information, see [Use AWS CodeBuild with a managed proxy server](run-codebuild-in-managed-proxy-server.md).  
Proxy configurations don't support VPC, Windows, or MacOS.  
**Default behavior**  
Defines the behavior of outgoing traffic.    
**Allow**  
Allows outgoing traffic to all destinations by default.  
**Deny**  
Denies outgoing traffic to all destinations by default.  
**Proxy rules**  
Specifies destination domains or IPs to allow or deny network access control to.