There are three types of rules that you will come in contact with when creating or editing a custom field, namely:
common rules (for all data)
These rule types differ in what data they are applied to, but also what hierarchy they have - meaning in what order the rules will be executed. Keep in mind, all rules are executed from top to bottom. This lets you specify in what order the rules should be executed.
The picture below shows where you'll find the different rules when editing or creating a custom field. The green area shows the section for adding a platform-specific rule, the blue area shows the section for adding a common rule (for all data sources) and the red area shows the section for the already existing standard rules.
Starting with the platform-specific rules, these rules are only applied to data from one specific data source type and can be distinguished in the interface by being fenced by a light grey box. What data source type data the rule will be applied to is indicated by the selected platform on top of the light grey box. Clicking one of the "+ Rule" buttons under the Bing section in the picture above would in other words result in creating a rule that will only be applied to Bing-data. In addition, platform-specific rules are having the highest priority when Funnel labels the data, and will in that way override any overlapping common rule or standard rule.
Common rules (For all Data Sources)
Moving on to common rules, these rules are applied to ANY data source type and are evaluated after the platform-specific rules. Rules specified in this section are best used when your data is having some common theme that you want to apply when grouping data. Such an example can be seen in the screenshot above for the default custom dimension Media Type. There we specify that for all data from all sources, if Campaign contains "Social", the value of Media type should be set to "Social".
This means that instead of writing the same rule over and over again for each concerned data source type, you just have to write the rule once. Handy, right?
Lastly, we have standard rules. These rules are predefined by Funnel and can't be removed from a custom field. However, they have the lowest priority when the rules are being executed and can therefore be easily overridden by common rules or platform-specific rules.