The optional Pro Grid Filler has the world's best filling engine with advanced options. It also lets you do semi-automated filling where you can choose each word as you go along.
In addition there is a feature to fit theme words into grids before filling around them.
This feature allows you to choose each word as it is placed into the grid, giving you full control. Words are sorted by how 'good' they are. If inserting a word would then force you to use some other word elsewhere this is clearly shown.
You can also look up words in dictionaries, insert clues, and view 'forced choices' - words you are forced to choose from later on if you choose a particular word now.
The program automatically suggests which word slot to fill next, but you can click in the grid to direct the fill to a different slot.
This feature will take a short list of theme words, then scan all the available grid patterns to find ones where they will fit. You can then view the possible arrangements and choose the one you like best.
There's also a button to fill around the theme words with words from the main word list. If it's not possible you are returned so you can choose another pattern.
There are options to choose whether words are placed randomly, in order, and/or symmetrically.
The Pro Grid Filler has extra advanced options in the Fill Grid window to give you control over the filling process. For example you can decide whether the fill should start at the word slot at the cursor position, or whether the program should determine where it thinks is the best place to start.
There is also a feature to avoid duplicating substrings - useful to avoid getting similar words in the grid.
You can fill from multiple word lists at once, or directly from a clue database.