OpenEXR is a high dynamic range raster file format, released as an open standard along with a set of software tools created by Industrial Light & Magic (ILM), under a free software license similar to the BSD license.
A full technical introduction of the format is available on openexr.org. The currently-supported version is OpenEXR 2.3.
Advantages of the EXR format are its support for up to 32-bit floating-point data (see TOP's Pixel Format parameter on the Common parameter page) and the ability to store an arbitrary number of channels.
The EXR files can be loaded into TouchDesigner with the Movie File In TOP which reads the first 4 channels as RGBA and the Point File In TOP which reads EXR files with any number of channels and custom channel mapping options.
EXR images can be saved out from TouchDesigner with (1) the Save Image... menu on TOPs, (2) the
.save() python method on TOPs, and (3) the Movie File Out TOP with Type "Image" or "Image Sequence".