Consent Mode.
Consent Mode is how your cookie banner tells Google's tags what each visitor has agreed to. It uses four signals: ad_storage, analytics_storage, ad_user_data and ad_personalization. The second version, called v2, added the last two, and Google's EU user consent policy, which covers visitors in the European Economic Area, the UK and Switzerland, is why it matters for ad measurement and remarketing.
Consent Mode only passes on the visitor's choice. It does not store it, so your banner has to do that. Done well, tags behave according to each choice, and done badly it can either record nothing or ignore what people said.
For example, if a visitor declines analytics cookies, GA4 does not set its own cookies on their device, but in the advanced setup it may still receive a basic anonymous signal that a visit happened.
See also Cookie consent
Modelled data.
Modelled data is GA4's estimate of the visits it could not observe because people declined cookies. It uses what it learned from visitors who did consent to fill the gap in the totals.
It only works if consent mode is set up in its advanced form, and only once there is enough data. Google sets minimum volumes: at least 1,000 events a day from visitors who declined, for at least 7 days, plus at least 1,000 daily users who agreed on at least 7 of the previous 28 days. Meeting them does not guarantee modelling, so a small website may never receive it. The figures are then simply those who agreed.
For example, a site with 1,000 recorded users may really have had more, because some visitors declined. Modelling can estimate those, but a very small site would normally see only the 1,000 observed.
See also Data gaps
Cross domain measurement.
Cross domain measurement lets GA4 treat a visit that moves between two of your websites, such as your main site and a separate booking or payment page, as one journey. It works by adding a short code called _gl to the link.
Without it, the customer appears as a brand new visitor on the second site and the sale is credited to a referral from your own website. It is fixed by listing both domains in the data stream settings.
For example, a hotel's website sends guests to a booking engine on another domain. Without cross domain measurement, 100 bookings may all be credited to 'referral', hiding the Google search that actually brought those guests.
Internal traffic filters.
An internal traffic filter removes visits from your own team, such as staff browsing the site or us testing it. You tell GA4 which IP addresses are yours, create the filter, and switch it from testing to active.
Without it, your own visits are counted as customers, which inflates sessions and can look like interest that isn't there. The filter only works from the day it is switched on, and it cannot recover earlier data.
For example, if a team of five checks the website daily and creates 40 of 500 monthly sessions, that is 8 per cent of the traffic that was never a customer.
Unwanted referrals.
An unwanted referral is a website that GA4 treats as the source of a visit but which really just sent your own customer back, such as a payment provider after checkout. GA4 lets you list these so they are not counted as a new source.
If they are not listed, a sale can be credited to the payment page rather than to the ad, search or email that really brought the customer, which hides your best sources.
See also Referral
Reporting identity.
Reporting identity is the setting that decides how GA4 recognises the same person across devices and sessions. The Blended option uses signed-in IDs where it has them, then device IDs, and then modelling.
Changing it can change your user counts, and it also affects whether thresholding applies. It rarely needs changing, but it is a good first question when two reports show different user numbers.