Tcl is a very simple programming language.

If you have programmed before, you can learn enough to write Tcl programs within an hour.  Jump to the Example Scripts at the end of this Help Book for some realistic applications. You can copy and paste into the WebAccess Script editor.

Tcl is a high-level scripting language.

Tcl is simple to use, but powerful in capabilities. You will write a lot less code to get the job done, especially when compared to C applications. It is similar in power to C applications.  There are email programs and web servers written in Tcl.

Tcl is interpreted.

You execute your code directly, without compiling and linking. You do have to "Download" your script to the SCADA node. If using a screen script on a client, the client has to also refresh his page.

Tcl is an open source standard.

You can get the sources of sample programs for free over the Internet. You can browse the Internet and find many more examples, user guides, etc.

Tcl is a standard programming language

It is not unique to WebAccess.  You can re-use what you learn here.  There may be users in your organization already familiar with Tcl.