How custom parameters work
Google Ads lets you invent your own URL values as well as use the ones it supplies. A custom parameter has a name you choose and a value you type, and it is referenced inside a tracking template or a final URL suffix using braces with a leading underscore. When somebody clicks the ad, Google swaps the reference for the value you stored and the landing page receives the finished address.
They can be defined at account, campaign, ad group, keyword or ad level. A value set lower down replaces the one above it, so a campaign can carry a different label from the rest of the account without you rewriting the template. Google allows a small fixed number of them per account, which is enough for labelling but not enough to build a database inside your URLs.
The distinction that matters is who fills in the value. ValueTrack parameters are filled in by Google at click time from what it knows about the auction. Custom parameters are filled in by you, in advance, and stay the same until you change them.
Why custom parameters matter
They carry the facts Google has no way of knowing. An internal client or branch code, a promotion name, a product line, the agency or staff member responsible for a campaign, a market label that does not match Google’s own geography — all of these are things you know and the platform does not.
That becomes valuable when the click leaves Google’s world. If the label travels into your analytics, your form and your CRM, you can group results the way your business actually thinks about them rather than the way the ad account happens to be structured. For a business selling in several countries from one account, a clean market label is often worth more than any extra report inside Google Ads.
Common mistakes with custom parameters
The first is reaching for one when a ValueTrack parameter already exists. Device, match type, network and campaign identity are supplied automatically, and hand-typing them creates values that quietly go stale.
The second is treating the parameter as a store for anything sensitive. A URL is written into server logs, browser history and analytics reports, so a customer name, an email address or an internal price should never sit in one.
The third is maintenance. A value set at ad group level is easy to forget when the ad group is copied, renamed or paused, and nothing in the interface warns you that a reference now resolves to nothing. Long, unnecessary parameters also lengthen the address, which does no favours to pages already carrying redirects.
How to act on it
Decide the short list of labels your reporting genuinely needs, agree what each value looks like, and write that down before anyone touches the account. Consistent lower-case values with no spaces save hours later, because analytics tools treat a capitalised value as a separate thing.
Set them as high in the account as the label allows, test a live click, and confirm the value survives all the way into whatever system you will read it in. Review them whenever campaigns are restructured, since a custom parameter is only as accurate as the last time somebody checked it.