With Nav 2013 a new License Model was introduced. Especially the new Limited User License is a bit hard to understand. First of all: full read access is permitted. But … write access is very strong limited. It’s said, that within a write transaction 3 tables are allowed to access, But … it’s not described, how to do that. That is not easy to handle, especially for developers. 😦  But ok. It would be interesting, how the Microsoft Developers themselves handle that limitation.
What does that mean at the end? When writing your own code, always check your list of used tables. A maximum of only 3 tables, you use, might not be listed in “Appendix A”, within your code snippet!
At least there are a couple of tables, that can be used anyway. Following is the original text out from the “Dynamics ERP Perpetual License Guide”.

Limited User licenses for Microsoft Dynamics NAV 2013 provide users full read, but limited write capabilities to all of the licensed solution functionality through any and all modes of access including the Windows Client. The Limited User license is designed to give customers a lower-cost alternative for extending ERP to users who only need to perform the following tasks:

• “Read access” to data contained in the ERP solution through any client accessing the application via the Microsoft Dynamics NAV Application Programming Interface (API) including but not limited to the User Interface Web Services for Microsoft Dynamics NAV
• “Write access” via the Timesheet functionality
• “Write access” through any client accessing the ERP solution via the Microsoft Dynamics NAV API to a maximum of 3 table objects in any range number with the following exceptions:
  • limited users are not allowed to write to any of the following tables: General Ledger (table 17), Permission Set (table 2000000004), Permission (table 2000000005) or Access Control (table 2000000053) and
  • any table described in Appendix A as “Limited User Included Tables” do not count towards the 3 table objects

APPENDIX A – Limited User Included Tables
The following tables do not count towards the maximum of 3 permitted for Limited Users in Microsoft
Dynamics NAV. Last updated 20 November 2012 (11 tables added)

Source: Page 6 and Appendix A in Dynamics ERP Perpetual Licensing Guide.