In Bitrix24, there are 2 objects that are meant to contain information about people: Leads and Contacts. There are important functional differences, too. The concept is that a Lead could be called a “potential” — meaning that either: a) you have limited information about their needs, or b) they have not purchased anything yet. So really a Lead is a first stage and Contact is a second stage. You might not need to use Leads at all, or very rarely.
For sure use Leads if: you want to keep early-stage potentials separate from active clients. So if you buy a list of 10,000 names or email addresses — those are Leads.
Don’t bother with Leads if: you have a small number of potentials and you have a good amount of information about each one when they are first added to the CRM.
The main functional difference is that Leads have Stages — meaning you can see where they are in a given process. Contacts, in turn, can be associated with Deals and Companies — there is a “Deals” and “Company” tab in the Contact detail page that will show you all Deals and the Company with which that Contact is associated.
Once a Lead is converted using the conversion process, then you should only work with the Contact that has been created from the conversion process. Don’t work with the Lead anymore – it will only create confusion.