When selecting a custom dimension in Funnel or in a Google Analytics report you will only see values for hits where that custom dimension has been set. For example if you assign a "Username" custom dimension to users that have logged in to you site you will not see data for users that haven't logged in.

When selecting multiple custom dimensions only the rows with values for all custom dimensions will be included. For example if you look at the "Username" custom dimension and "First time visitor" then you will only see data for users that have logged in and who visited the site for the first time.

As a result of how data is being filtered in Google Analytics for custom dimensions historical data before the custom dimension was created will not be available when including the custom dimension in the Funnel data source.

Documentation from Google Analytics can be found here.

Examples

ga('send', 'event', {
  eventCategory: 'category 1',
  eventAction: 'action 1',
  dimension1: 'Dimension value'
});

Example of sending custom dimension 1 with an event using analytics.js.

ga('set', 'dimension1', 'Dimension value');
ga('send', 'event');

Example of setting custom dimension 1 on the tracker so that every pageview and event hit receives the custom dimension using analytics.js.