Ticket allocation strategy:

You should decide how to distribute available seats between categories. The default is to create one or more dynamic categories. If unsure, please select "Grow dynamically"

Grow dynamically

This category doesn't allocate seats exclusively, instead it shares them with other dynamic categories, if any. This is useful if you have only one category or if you have different ways to sell the same seat.
Limited seats

Select this option if you want to allocate exclusively a fixed number of seats to this category. This could be useful if you want to create a special price, like a "Super early bird".

Category visibility:

Categories can be public (that is, accessible to everybody) or restricted (accessible after inserting an unique discount code)

Public

Everyone can buy/get a ticket for this category. This is the default option.
Restricted

Only a restricted group of people can access and get tickets for this category. Once you create a restricted category, the system will generate a set of unique codes (one code per seat) that can be used to reserve a ticket for this category.
Restricted categories are not compatible with "Grow dynamically" ticket allocation strategy

Restrict check-in to a specific timeslot

from

Custom ticket validity (will be displayed in ticket PDF, email and calendar invitation)

from

Access Code:

You can define an access code for each category, either public or restricted, and then distribute the resulting URL.

If someone access the URL, a new 1-ticket reservation will be created for this category.