Sudoku notation
How cells, boxes, eliminations and pencil marks are written in guides and in the solver.
Cells and boxes
Guides and the solver write a cell as rXcY: row X, column Y, both counted from 1. So r1c6 is the first row, sixth column. A box is written bN (b3 is the top right box). Rows are numbered top to bottom, columns left to right, boxes in the same reading order.
Placing and removing digits
Placing a digit is written with an equals sign: r5c5=3, a 3 in the center cell. Removing a candidate uses <>: r1c6<>4 says that 4 is not a candidate of that cell. Several cells in a row of the notation (r1c6, r2c6<>4) mean the same removal in each. A hint in the solver reads, for example, Locked Candidates Type 1 (Pointing): 4 in b3 ⇒ r1c6<>4.
Chains
In chain notation, a strong link is shown with = and a weak link with -, so 4r1c1 = 4r1c7 - 5r1c7 reads: in row 1 the 4 goes in c1 or c7, and in cell r1c7 a 4 rules out a 5. The solver draws strong links as solid lines and weak links as dashed lines.
Pencil marks
On paper, candidates are written small in the cell. Writing every candidate in every cell is called full notation; it is thorough and cluttered. Snyder notation (the name varies by author) writes a digit in a box only where it fits in exactly two cells of that box: a short list that already shows where pairs and locked candidates are hiding.
In the app, Notes mode lets you write your own marks, in whatever style you like, and Settings → Show candidates shows all the candidates the solver keeps. Your marks and the solver's candidates are separate, so you can compare them.
Writing a puzzle as text
A puzzle is commonly written as 81 characters, row by row, with a dot or a zero for each empty cell. The app copies and opens that format, and several others; see the manual.