The following properties can all be used in a Check-In solution.
Usually, you'll only be needing the standard fields, but for some custom solutions, the special fields can be useful.

Standard fields

client.Name

client.Description

client.RFID

client.RFIDReversed

client.MailDomain

client.Disabled

client.Tags (as list)

client.TagsString (as text)

Special fields

client.EnterpriseData.CustomerNumber

client.EnterpriseData.Organization

client.EnterpriseData.ContactName

client.EnterpriseData.ContactTitle

client.EnterpriseData.TaxNumber

client.EnterpriseData.Address1

client.EnterpriseData.Address2

client.EnterpriseData.PostalCode

client.EnterpriseData.City

client.EnterpriseData.State

client.EnterpriseData.CountryCode

client.EnterpriseData.Email

client.EnterpriseData.Phone

client.EnterpriseData.Mobile

client.EnterpriseData.HasLunchPlan

Group can also be used as a property instead of 'client'.

group.Name

group.ContactName

group.LogoExtension

group.VAT

group.Address1

group.Address2

group.Postal

group.City

group.Region

group.CountryCode

group.Phone

group.Email

group.MailDomain

group.Description

group.CustomerNumber