An attribute is a custom field you keep on a contact — Region, Customer Type, whatever your business actually tracks. Registering one here is what makes it a known field rather than a note someone typed once.
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Why register an attribute
Values can be put on a contact whether or not the field is registered here, so the natural question is what this list is for. Registering a name buys you three specific things. It shows up in the picker when anyone edits a contact, so your team chooses Region from a list instead of five people inventing region, Region and REGION. It becomes available in Automations → Rules, both as a trigger — when this attribute changes, do something — and as a condition to filter on; an unregistered field can't do either, which is the strongest reason to come here first. And if you give it options, it turns into a dropdown in the inbox. Names are yours to choose, up to 50 attributes.
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Free text, or a dropdown in the inbox
The New dialog asks for a name and, optionally, a list of Dropdown options — and that second field is the one that does something bigger than it looks. Leave it empty and the attribute is free text: anyone can type anything into it. Add options and that attribute becomes a dropdown on the contact panel of every conversation, in the Sales section, ready to set while an agent is mid-chat. That's how the Lead Stage field you already have works — New Lead, Contacted, Qualified and the rest are just options on an ordinary attribute. It's the difference between a field someone fills in later and one your agents actually keep current, so it's worth adding options to anything you want tracked during a conversation. Type each option and press Enter; repeats are ignored, and clearing them all turns the attribute back into free text.
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Renaming and removing
Renaming is safe, and more thorough than you'd expect: change Region to Territory and every contact already carrying it is updated to match, so nothing is stranded under the old name. Deleting asks you to confirm first, and it's worth reading that prompt — the attribute disappears from the registry and from every picker, which means any rule built on it stops having a field to watch. If a rule depends on an attribute, retire the rule before the field. If an attribute keeps reappearing after you delete it, that's not a glitch: Lead Stage ships with the product and is recreated automatically.