As there are a lot of game systems around and each one has its own specific set of rules about building an army list. To be able to handle all possible game systems ArmyCalc uses 'Rulesets'.
You can think about 'Ruleset' as a set of data that contains all necessary information about certain army for certain game system. It contains characteristics of all available units and additional items you can equip to this units with. It can contain media files like thumbnails for units. If a game system have complicated rules about building an army, 'Ruleset' can also contain scripts to validate this rules. If you need some fancy look for army lists PDFs you can also add a script that will provide a custom display.
'Ruleset' can be exported to TWR - an open file format. This is a common file format for all ArmyCalc applications. Check out File Formats document for more information.
In My Rulesets section you can manage your Rulesets collection. All Rulesets here will be also visible when creating new army in My Armies. You can create new, empty Ruleset here or upload it from TWR file if you already have one.
After creating new file it will be displayed as 'Empty Ruleset'. Click 'edit' link next to it.
While editing a Ruleset you can change all its basic properties and manage files it contains. Each Ruleset should contain at least one Units XML file. To find out how the units files should like check File Formats - 'Units XMLs' section or see Examples.
You do not need armyclac.com or any special tools to build TWR files. You will just need a text file editor and any archiving application than supports *.tar.gz format. To do this you need to know general structure of TWR file and follow instructions in File Formats.
TWR is an opened, 'free' file format. Everyone is free to develop applications and tools using it.
You can check Simple ArmyCalc Ruleset Builder external tool created by ArmyCalc user. It converts CSV files created in Exel or any text editor to TWR.
Fastest way to get to know how XML's and scripts inside a Ruleset work is to check out Example Ruleset. This Ruleset contains all TWR features.
Besides, content of all published Rulesets that can be found under My Rulesets is available to read. So if you wonder how certain thing is done just check it.
If you have any questions do not hesitate to ask them on Creating Rulesets forum.