# Replacement cross-reference lookups
## Introduction
Having [chosen to configure cross-reference lookup replacements for a virtual environment](https://doc.wearepatchworks.com/product-documentation/virtual-environments/implementing-virtual-environments/configuring-a-virtual-environment-version/configuring-virtual-environment-replacements/..#configuring-replacements-for-a-virtual-environment-version), a new replacement rule is displayed, ready for you to configure:
Having defined the first rule, you can add more - you must create one replacement rule for every [cross-reference lookup table](https://doc.wearepatchworks.com/product-documentation/process-flows/cross-reference-lookups) that you need to replace in every process flow that will run in this environment.
## Defining replacement rules for a cross-reference lookup
{% stepper %}
{% step %}
**Select the required ORIGINAL cross-reference lookup**
Use the `original` dropdown list to select the first cross-reference lookup in a 'master' process flow that must be replaced when this process flow runs in the given virtual environment - for example:
{% hint style="info" %}
As you work through your replacement configuration, it's helpful to view related ['master' process flows](https://doc.wearepatchworks.com/product-documentation/virtual-environments/implementing-virtual-environments/creating-a-master-process-flow) in another browser tab. You can duplicate a Patchworks browser tab and navigate to a different page whenever needed.
{% endhint %}
{% endstep %}
{% step %}
**Select the required REPLACEMENT cross-reference lookup**
Use the `replacement` dropdown list to select the replacement cross-reference lookup that must be used when this process flow runs in the given virtual environment - for example:
{% endstep %}
{% step %}
**Confirm your selections**
Click the `create` button (associated with this replacement rule) to confirm your selections:
The rule is added:
{% hint style="info" %}
Once a rule is confirmed, it can't be edited. If you need to change a replacement rule, use the 'trash' icon to remove it and add a new one.
{% endhint %}
{% endstep %}
{% step %}
**Add more replacement rules for cross-reference lookups**
To add another replacement rule for a cross-reference lookup, click the top-right `create` button:
Update the new rule as required, then click the associated `create` button to confirm your selections:
Repeat as many times as needed to define all required cross-reference lookup replacements for all process flows that will be running in this virtual environment.
{% hint style="info" %}
If you don't want to define ALL cross-reference lookup replacements now, that's OK. For example, you might choose to add the minimum number of replacements needed for [testing](https://doc.wearepatchworks.com/product-documentation/virtual-environments/implementing-virtual-environments/testing-a-process-flow-against-a-virtual-environment), then define remaining rules (in a [new draft](https://doc.wearepatchworks.com/product-documentation/virtual-environments/implementing-virtual-environments/configuring-a-virtual-environment-version/understanding-virtual-environment-versions)) later, after initial testing is complete.
{% endhint %}
{% endstep %}
{% endstepper %}
